Kernel/linux-next
1
0
Fork 0
mirror of https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git synced 2024-12-29 17:22:07 +00:00
Code Issues Actions Packages Projects Releases Wiki Activity

documentation: add IPE documentation

Browse Source 
Add IPE's admin and developer documentation to the kernel tree.

Co-developed-by: Fan Wu <wufan@linux.microsoft.com>
Signed-off-by: Deven Bowers <deven.desai@linux.microsoft.com>
Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
This commit is contained in:
Deven Bowers 2024-08-02 23:08:33 -07:00 committed by Paul Moore
parent 10ca05a760
commit ac6731870e
6 changed files with 1255 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
Documentation/admin-guide/LSM/index.rst
View File

@ -47,3 +47,4 @@ subdirectories.
tomoyo
Yama
SafeSetID
ipe

790
Documentation/admin-guide/LSM/ipe.rst Normal file
View File

@ -0,0 +1,790 @@
.. SPDX-License-Identifier: GPL-2.0
Integrity Policy Enforcement (IPE)
==================================
.. NOTE::
This is the documentation for admins, system builders, or individuals
attempting to use IPE. If you're looking for more developer-focused
documentation about IPE please see :doc:`the design docs </security/ipe>`.
Overview
--------
Integrity Policy Enforcement (IPE) is a Linux Security Module that takes a
complementary approach to access control. Unlike traditional access control
mechanisms that rely on labels and paths for decision-making, IPE focuses
on the immutable security properties inherent to system components. These
properties are fundamental attributes or features of a system component
that cannot be altered, ensuring a consistent and reliable basis for
security decisions.
To elaborate, in the context of IPE, system components primarily refer to
files or the devices these files reside on. However, this is just a
starting point. The concept of system components is flexible and can be
extended to include new elements as the system evolves. The immutable
properties include the origin of a file, which remains constant and
unchangeable over time. For example, IPE policies can be crafted to trust
files originating from the initramfs. Since initramfs is typically verified
by the bootloader, its files are deemed trustworthy; "file is from
initramfs" becomes an immutable property under IPE's consideration.
The immutable property concept extends to the security features enabled on
a file's origin, such as dm-verity or fs-verity, which provide a layer of
integrity and trust. For example, IPE allows the definition of policies
that trust files from a dm-verity protected device. dm-verity ensures the
integrity of an entire device by providing a verifiable and immutable state
of its contents. Similarly, fs-verity offers filesystem-level integrity
checks, allowing IPE to enforce policies that trust files protected by
fs-verity. These two features cannot be turned off once established, so
they are considered immutable properties. These examples demonstrate how
IPE leverages immutable properties, such as a file's origin and its
integrity protection mechanisms, to make access control decisions.
For the IPE policy, specifically, it grants the ability to enforce
stringent access controls by assessing security properties against
reference values defined within the policy. This assessment can be based on
the existence of a security property (e.g., verifying if a file originates
from initramfs) or evaluating the internal state of an immutable security
property. The latter includes checking the roothash of a dm-verity
protected device, determining whether dm-verity possesses a valid
signature, assessing the digest of a fs-verity protected file, or
determining whether fs-verity possesses a valid built-in signature. This
nuanced approach to policy enforcement enables a highly secure and
customizable system defense mechanism, tailored to specific security
requirements and trust models.
To enable IPE, ensure that ``CONFIG_SECURITY_IPE`` (under
:menuselection:`Security -> Integrity Policy Enforcement (IPE)`) config
option is enabled.
Use Cases
---------
IPE works best in fixed-function devices: devices in which their purpose
is clearly defined and not supposed to be changed (e.g. network firewall
device in a data center, an IoT device, etcetera), where all software and
configuration is built and provisioned by the system owner.
IPE is a long-way off for use in general-purpose computing: the Linux
community as a whole tends to follow a decentralized trust model (known as
the web of trust), which IPE has no support for it yet. Instead, IPE
supports PKI (public key infrastructure), which generally designates a
set of trusted entities that provide a measure of absolute trust.
Additionally, while most packages are signed today, the files inside
the packages (for instance, the executables), tend to be unsigned. This
makes it difficult to utilize IPE in systems where a package manager is
expected to be functional, without major changes to the package manager
and ecosystem behind it.
The digest_cache LSM [#digest_cache_lsm]_ is a system that when combined with IPE,
could be used to enable and support general-purpose computing use cases.
Known Limitations
-----------------
IPE cannot verify the integrity of anonymous executable memory, such as
the trampolines created by gcc closures and libffi (<3.4.2), or JIT'd code.
Unfortunately, as this is dynamically generated code, there is no way
for IPE to ensure the integrity of this code to form a trust basis.
IPE cannot verify the integrity of programs written in interpreted
languages when these scripts are invoked by passing these program files
to the interpreter. This is because the way interpreters execute these
files; the scripts themselves are not evaluated as executable code
through one of IPE's hooks, but they are merely text files that are read
(as opposed to compiled executables) [#interpreters]_.
Threat Model
------------
IPE specifically targets the risk of tampering with user-space executable
code after the kernel has initially booted, including the kernel modules
loaded from userspace via ``modprobe`` or ``insmod``.
To illustrate, consider a scenario where an untrusted binary, possibly
malicious, is downloaded along with all necessary dependencies, including a
loader and libc. The primary function of IPE in this context is to prevent
the execution of such binaries and their dependencies.
IPE achieves this by verifying the integrity and authenticity of all
executable code before allowing them to run. It conducts a thorough
check to ensure that the code's integrity is intact and that they match an
authorized reference value (digest, signature, etc) as per the defined
policy. If a binary does not pass this verification process, either
because its integrity has been compromised or it does not meet the
authorization criteria, IPE will deny its execution. Additionally, IPE
generates audit logs which may be utilized to detect and analyze failures
resulting from policy violation.
Tampering threat scenarios include modification or replacement of
executable code by a range of actors including:
- Actors with physical access to the hardware
- Actors with local network access to the system
- Actors with access to the deployment system
- Compromised internal systems under external control
- Malicious end users of the system
- Compromised end users of the system
- Remote (external) compromise of the system
IPE does not mitigate threats arising from malicious but authorized
developers (with access to a signing certificate), or compromised
developer tools used by them (i.e. return-oriented programming attacks).
Additionally, IPE draws hard security boundary between userspace and
kernelspace. As a result, kernel-level exploits are considered outside
the scope of IPE and mitigation is left to other mechanisms.
Policy
------
IPE policy is a plain-text [#devdoc]_ policy composed of multiple statements
over several lines. There is one required line, at the top of the
policy, indicating the policy name, and the policy version, for
instance::
policy_name=Ex_Policy policy_version=0.0.0
The policy name is a unique key identifying this policy in a human
readable name. This is used to create nodes under securityfs as well as
uniquely identify policies to deploy new policies vs update existing
policies.
The policy version indicates the current version of the policy (NOT the
policy syntax version). This is used to prevent rollback of policy to
potentially insecure previous versions of the policy.
The next portion of IPE policy are rules. Rules are formed by key=value
pairs, known as properties. IPE rules require two properties: ``action``,
which determines what IPE does when it encounters a match against the
rule, and ``op``, which determines when the rule should be evaluated.
The ordering is significant, a rule must start with ``op``, and end with
``action``. Thus, a minimal rule is::
op=EXECUTE action=ALLOW
This example will allow any execution. Additional properties are used to
assess immutable security properties about the files being evaluated.
These properties are intended to be descriptions of systems within the
kernel that can provide a measure of integrity verification, such that IPE
can determine the trust of the resource based on the value of the property.
Rules are evaluated top-to-bottom. As a result, any revocation rules,
or denies should be placed early in the file to ensure that these rules
are evaluated before a rule with ``action=ALLOW``.
IPE policy supports comments. The character '#' will function as a
comment, ignoring all characters to the right of '#' until the newline.
The default behavior of IPE evaluations can also be expressed in policy,
through the ``DEFAULT`` statement. This can be done at a global level,
or a per-operation level::
# Global
DEFAULT action=ALLOW
# Operation Specific
DEFAULT op=EXECUTE action=ALLOW
A default must be set for all known operations in IPE. If you want to
preserve older policies being compatible with newer kernels that can introduce
new operations, set a global default of ``ALLOW``, then override the
defaults on a per-operation basis (as above).
With configurable policy-based LSMs, there's several issues with
enforcing the configurable policies at startup, around reading and
parsing the policy:
1. The kernel *should* not read files from userspace, so directly reading
the policy file is prohibited.
2. The kernel command line has a character limit, and one kernel module
should not reserve the entire character limit for its own
configuration.
3. There are various boot loaders in the kernel ecosystem, so handing
off a memory block would be costly to maintain.
As a result, IPE has addressed this problem through a concept of a "boot
policy". A boot policy is a minimal policy which is compiled into the
kernel. This policy is intended to get the system to a state where
userspace is set up and ready to receive commands, at which point a more
complex policy can be deployed via securityfs. The boot policy can be
specified via ``SECURITY_IPE_BOOT_POLICY`` config option, which accepts
a path to a plain-text version of the IPE policy to apply. This policy
will be compiled into the kernel. If not specified, IPE will be disabled
until a policy is deployed and activated through securityfs.
Deploying Policies
~~~~~~~~~~~~~~~~~~
Policies can be deployed from userspace through securityfs. These policies
are signed through the PKCS#7 message format to enforce some level of
authorization of the policies (prohibiting an attacker from gaining
unconstrained root, and deploying an "allow all" policy). These
policies must be signed by a certificate that chains to the
``SYSTEM_TRUSTED_KEYRING``. With openssl, the policy can be signed by::
openssl smime -sign \
-in "$MY_POLICY" \
-signer "$MY_CERTIFICATE" \
-inkey "$MY_PRIVATE_KEY" \
-noattr \
-nodetach \
-nosmimecap \
-outform der \
-out "$MY_POLICY.p7b"
Deploying the policies is done through securityfs, through the
``new_policy`` node. To deploy a policy, simply cat the file into the
securityfs node::
cat "$MY_POLICY.p7b" > /sys/kernel/security/ipe/new_policy
Upon success, this will create one subdirectory under
``/sys/kernel/security/ipe/policies/``. The subdirectory will be the
``policy_name`` field of the policy deployed, so for the example above,
the directory will be ``/sys/kernel/security/ipe/policies/Ex_Policy``.
Within this directory, there will be seven files: ``pkcs7``, ``policy``,
``name``, ``version``, ``active``, ``update``, and ``delete``.
The ``pkcs7`` file is read-only. Reading it returns the raw PKCS#7 data
that was provided to the kernel, representing the policy. If the policy being
read is the boot policy, this will return ``ENOENT``, as it is not signed.
The ``policy`` file is read only. Reading it returns the PKCS#7 inner
content of the policy, which will be the plain text policy.
The ``active`` file is used to set a policy as the currently active policy.
This file is rw, and accepts a value of ``"1"`` to set the policy as active.
Since only a single policy can be active at one time, all other policies
will be marked inactive. The policy being marked active must have a policy
version greater or equal to the currently-running version.
The ``update`` file is used to update a policy that is already present
in the kernel. This file is write-only and accepts a PKCS#7 signed
policy. Two checks will always be performed on this policy: First, the
``policy_names`` must match with the updated version and the existing
version. Second the updated policy must have a policy version greater than
or equal to the currently-running version. This is to prevent rollback attacks.
The ``delete`` file is used to remove a policy that is no longer needed.
This file is write-only and accepts a value of ``1`` to delete the policy.
On deletion, the securityfs node representing the policy will be removed.
However, delete the current active policy is not allowed and will return
an operation not permitted error.
Similarly, writing to both ``update`` and ``new_policy`` could result in
bad message(policy syntax error) or file exists error. The latter error happens
when trying to deploy a policy with a ``policy_name`` while the kernel already
has a deployed policy with the same ``policy_name``.
Deploying a policy will *not* cause IPE to start enforcing the policy. IPE will
only enforce the policy marked active. Note that only one policy can be active
at a time.
Once deployment is successful, the policy can be activated, by writing file
``/sys/kernel/security/ipe/policies/$policy_name/active``.
For example, the ``Ex_Policy`` can be activated by::
echo 1 > "/sys/kernel/security/ipe/policies/Ex_Policy/active"
From above point on, ``Ex_Policy`` is now the enforced policy on the
system.
IPE also provides a way to delete policies. This can be done via the
``delete`` securityfs node,
``/sys/kernel/security/ipe/policies/$policy_name/delete``.
Writing ``1`` to that file deletes the policy::
echo 1 > "/sys/kernel/security/ipe/policies/$policy_name/delete"
There is only one requirement to delete a policy: the policy being deleted
must be inactive.
.. NOTE::
If a traditional MAC system is enabled (SELinux, apparmor, smack), all
writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``.
Modes
~~~~~
IPE supports two modes of operation: permissive (similar to SELinux's
permissive mode) and enforced. In permissive mode, all events are
checked and policy violations are logged, but the policy is not really
enforced. This allows users to test policies before enforcing them.
The default mode is enforce, and can be changed via the kernel command
line parameter ``ipe.enforce=(0|1)``, or the securityfs node
``/sys/kernel/security/ipe/enforce``.
.. NOTE::
If a traditional MAC system is enabled (SELinux, apparmor, smack, etcetera),
all writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``.
Audit Events
~~~~~~~~~~~~
1420 AUDIT_IPE_ACCESS
^^^^^^^^^^^^^^^^^^^^^
Event Examples::
type=1420 audit(1653364370.067:61): ipe_op=EXECUTE ipe_hook=MMAP enforcing=1 pid=2241 comm="ld-linux.so" path="/deny/lib/libc.so.6" dev="sda2" ino=14549020 rule="DEFAULT action=DENY"
type=1300 audit(1653364370.067:61): SYSCALL arch=c000003e syscall=9 success=no exit=-13 a0=7f1105a28000 a1=195000 a2=5 a3=812 items=0 ppid=2219 pid=2241 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="ld-linux.so" exe="/tmp/ipe-test/lib/ld-linux.so" subj=unconfined key=(null)
type=1327 audit(1653364370.067:61): 707974686F6E3300746573742F6D61696E2E7079002D6E00
type=1420 audit(1653364735.161:64): ipe_op=EXECUTE ipe_hook=MMAP enforcing=1 pid=2472 comm="mmap_test" path=? dev=? ino=? rule="DEFAULT action=DENY"
type=1300 audit(1653364735.161:64): SYSCALL arch=c000003e syscall=9 success=no exit=-13 a0=0 a1=1000 a2=4 a3=21 items=0 ppid=2219 pid=2472 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="mmap_test" exe="/root/overlake_test/upstream_test/vol_fsverity/bin/mmap_test" subj=unconfined key=(null)
type=1327 audit(1653364735.161:64): 707974686F6E3300746573742F6D61696E2E7079002D6E00
This event indicates that IPE made an access control decision; the IPE
specific record (1420) is always emitted in conjunction with a
``AUDITSYSCALL`` record.
Determining whether IPE is in permissive or enforced mode can be derived
from ``success`` property and exit code of the ``AUDITSYSCALL`` record.
Field descriptions:
+-----------+------------+-----------+---------------------------------------------------------------------------------+
| Field | Value Type | Optional? | Description of Value |
+===========+============+===========+=================================================================================+
| ipe_op | string | No | The IPE operation name associated with the log |
+-----------+------------+-----------+---------------------------------------------------------------------------------+
| ipe_hook | string | No | The name of the LSM hook that triggered the IPE event |
+-----------+------------+-----------+---------------------------------------------------------------------------------+
| enforcing | integer | No | The current IPE enforcing state 1 is in enforcing mode, 0 is in permissive mode |
+-----------+------------+-----------+---------------------------------------------------------------------------------+
| pid | integer | No | The pid of the process that triggered the IPE event. |
+-----------+------------+-----------+---------------------------------------------------------------------------------+
| comm | string | No | The command line program name of the process that triggered the IPE event |
+-----------+------------+-----------+---------------------------------------------------------------------------------+
| path | string | Yes | The absolute path to the evaluated file |
+-----------+------------+-----------+---------------------------------------------------------------------------------+
| ino | integer | Yes | The inode number of the evaluated file |
+-----------+------------+-----------+---------------------------------------------------------------------------------+
| dev | string | Yes | The device name of the evaluated file, e.g. vda |
+-----------+------------+-----------+---------------------------------------------------------------------------------+
| rule | string | No | The matched policy rule |
+-----------+------------+-----------+---------------------------------------------------------------------------------+
1421 AUDIT_IPE_CONFIG_CHANGE
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Event Example::
type=1421 audit(1653425583.136:54): old_active_pol_name="Allow_All" old_active_pol_version=0.0.0 old_policy_digest=sha256:E3B0C44298FC1C149AFBF4C8996FB92427AE41E4649B934CA495991B7852B855 new_active_pol_name="boot_verified" new_active_pol_version=0.0.0 new_policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB auid=4294967295 ses=4294967295 lsm=ipe res=1
type=1300 audit(1653425583.136:54): SYSCALL arch=c000003e syscall=1 success=yes exit=2 a0=3 a1=5596fcae1fb0 a2=2 a3=2 items=0 ppid=184 pid=229 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="python3" exe="/usr/bin/python3.10" key=(null)
type=1327 audit(1653425583.136:54): PROCTITLE proctitle=707974686F6E3300746573742F6D61696E2E7079002D66002E2
This event indicates that IPE switched the active poliy from one to another
along with the version and the hash digest of the two policies.
Note IPE can only have one policy active at a time, all access decision
evaluation is based on the current active policy.
The normal procedure to deploy a new policy is loading the policy to deploy
into the kernel first, then switch the active policy to it.
This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall.
Field descriptions:
+------------------------+------------+-----------+---------------------------------------------------+
| Field | Value Type | Optional? | Description of Value |
+========================+============+===========+===================================================+
| old_active_pol_name | string | Yes | The name of previous active policy |
+------------------------+------------+-----------+---------------------------------------------------+
| old_active_pol_version | string | Yes | The version of previous active policy |
+------------------------+------------+-----------+---------------------------------------------------+
| old_policy_digest | string | Yes | The hash of previous active policy |
+------------------------+------------+-----------+---------------------------------------------------+
| new_active_pol_name | string | No | The name of current active policy |
+------------------------+------------+-----------+---------------------------------------------------+
| new_active_pol_version | string | No | The version of current active policy |
+------------------------+------------+-----------+---------------------------------------------------+
| new_policy_digest | string | No | The hash of current active policy |
+------------------------+------------+-----------+---------------------------------------------------+
| auid | integer | No | The login user ID |
+------------------------+------------+-----------+---------------------------------------------------+
| ses | integer | No | The login session ID |
+------------------------+------------+-----------+---------------------------------------------------+
| lsm | string | No | The lsm name associated with the event |
+------------------------+------------+-----------+---------------------------------------------------+
| res | integer | No | The result of the audited operation(success/fail) |
+------------------------+------------+-----------+---------------------------------------------------+
1422 AUDIT_IPE_POLICY_LOAD
^^^^^^^^^^^^^^^^^^^^^^^^^^
Event Example::
type=1422 audit(1653425529.927:53): policy_name="boot_verified" policy_version=0.0.0 policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB auid=4294967295 ses=4294967295 lsm=ipe res=1
type=1300 audit(1653425529.927:53): arch=c000003e syscall=1 success=yes exit=2567 a0=3 a1=5596fcae1fb0 a2=a07 a3=2 items=0 ppid=184 pid=229 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="python3" exe="/usr/bin/python3.10" key=(null)
type=1327 audit(1653425529.927:53): PROCTITLE proctitle=707974686F6E3300746573742F6D61696E2E7079002D66002E2E
This record indicates a new policy has been loaded into the kernel with the policy name, policy version and policy hash.
This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall.
Field descriptions:
+----------------+------------+-----------+---------------------------------------------------+
| Field | Value Type | Optional? | Description of Value |
+================+============+===========+===================================================+
| policy_name | string | No | The policy_name |
+----------------+------------+-----------+---------------------------------------------------+
| policy_version | string | No | The policy_version |
+----------------+------------+-----------+---------------------------------------------------+
| policy_digest | string | No | The policy hash |
+----------------+------------+-----------+---------------------------------------------------+
| auid | integer | No | The login user ID |
+----------------+------------+-----------+---------------------------------------------------+
| ses | integer | No | The login session ID |
+----------------+------------+-----------+---------------------------------------------------+
| lsm | string | No | The lsm name associated with the event |
+----------------+------------+-----------+---------------------------------------------------+
| res | integer | No | The result of the audited operation(success/fail) |
+----------------+------------+-----------+---------------------------------------------------+
1404 AUDIT_MAC_STATUS
^^^^^^^^^^^^^^^^^^^^^
Event Examples::
type=1404 audit(1653425689.008:55): enforcing=0 old_enforcing=1 auid=4294967295 ses=4294967295 enabled=1 old-enabled=1 lsm=ipe res=1
type=1300 audit(1653425689.008:55): arch=c000003e syscall=1 success=yes exit=2 a0=1 a1=55c1065e5c60 a2=2 a3=0 items=0 ppid=405 pid=441 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=)
type=1327 audit(1653425689.008:55): proctitle="-bash"
type=1404 audit(1653425689.008:55): enforcing=1 old_enforcing=0 auid=4294967295 ses=4294967295 enabled=1 old-enabled=1 lsm=ipe res=1
type=1300 audit(1653425689.008:55): arch=c000003e syscall=1 success=yes exit=2 a0=1 a1=55c1065e5c60 a2=2 a3=0 items=0 ppid=405 pid=441 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=)
type=1327 audit(1653425689.008:55): proctitle="-bash"
This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall.
Field descriptions:
+---------------+------------+-----------+-------------------------------------------------------------------------------------------------+
| Field | Value Type | Optional? | Description of Value |
+===============+============+===========+=================================================================================================+
| enforcing | integer | No | The enforcing state IPE is being switched to, 1 is in enforcing mode, 0 is in permissive mode |
+---------------+------------+-----------+-------------------------------------------------------------------------------------------------+
| old_enforcing | integer | No | The enforcing state IPE is being switched from, 1 is in enforcing mode, 0 is in permissive mode |
+---------------+------------+-----------+-------------------------------------------------------------------------------------------------+
| auid | integer | No | The login user ID |
+---------------+------------+-----------+-------------------------------------------------------------------------------------------------+
| ses | integer | No | The login session ID |
+---------------+------------+-----------+-------------------------------------------------------------------------------------------------+
| enabled | integer | No | The new TTY audit enabled setting |
+---------------+------------+-----------+-------------------------------------------------------------------------------------------------+
| old-enabled | integer | No | The old TTY audit enabled setting |
+---------------+------------+-----------+-------------------------------------------------------------------------------------------------+
| lsm | string | No | The lsm name associated with the event |
+---------------+------------+-----------+-------------------------------------------------------------------------------------------------+
| res | integer | No | The result of the audited operation(success/fail) |
+---------------+------------+-----------+-------------------------------------------------------------------------------------------------+
Success Auditing
^^^^^^^^^^^^^^^^
IPE supports success auditing. When enabled, all events that pass IPE
policy and are not blocked will emit an audit event. This is disabled by
default, and can be enabled via the kernel command line
``ipe.success_audit=(0|1)`` or
``/sys/kernel/security/ipe/success_audit`` securityfs file.
This is *very* noisy, as IPE will check every userspace binary on the
system, but is useful for debugging policies.
.. NOTE::
If a traditional MAC system is enabled (SELinux, apparmor, smack, etcetera),
all writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``.
Properties
----------
As explained above, IPE properties are ``key=value`` pairs expressed in IPE
policy. Two properties are built-into the policy parser: 'op' and 'action'.
The other properties are used to restrict immutable security properties
about the files being evaluated. Currently those properties are:
'``boot_verified``', '``dmverity_signature``', '``dmverity_roothash``',
'``fsverity_signature``', '``fsverity_digest``'. A description of all
properties supported by IPE are listed below:
op
~~
Indicates the operation for a rule to apply to. Must be in every rule,
as the first token. IPE supports the following operations:
``EXECUTE``
Pertains to any file attempting to be executed, or loaded as an
executable.
``FIRMWARE``:
Pertains to firmware being loaded via the firmware_class interface.
This covers both the preallocated buffer and the firmware file
itself.
``KMODULE``:
Pertains to loading kernel modules via ``modprobe`` or ``insmod``.
``KEXEC_IMAGE``:
Pertains to kernel images loading via ``kexec``.
``KEXEC_INITRAMFS``
Pertains to initrd images loading via ``kexec --initrd``.
``POLICY``:
Controls loading policies via reading a kernel-space initiated read.
An example of such is loading IMA policies by writing the path
to the policy file to ``$securityfs/ima/policy``
``X509_CERT``:
Controls loading IMA certificates through the Kconfigs,
``CONFIG_IMA_X509_PATH`` and ``CONFIG_EVM_X509_PATH``.
action
~~~~~~
Determines what IPE should do when a rule matches. Must be in every
rule, as the final clause. Can be one of:
``ALLOW``:
If the rule matches, explicitly allow access to the resource to proceed
without executing any more rules.
``DENY``:
If the rule matches, explicitly prohibit access to the resource to
proceed without executing any more rules.
boot_verified
~~~~~~~~~~~~~
This property can be utilized for authorization of files from initramfs.
The format of this property is::
boot_verified=(TRUE|FALSE)
.. WARNING::
This property will trust files from initramfs(rootfs). It should
only be used during early booting stage. Before mounting the real
rootfs on top of the initramfs, initramfs script will recursively
remove all files and directories on the initramfs. This is typically
implemented by using switch_root(8) [#switch_root]_. Therefore the
initramfs will be empty and not accessible after the real
rootfs takes over. It is advised to switch to a different policy
that doesn't rely on the property after this point.
This ensures that the trust policies remain relevant and effective
throughout the system's operation.
dmverity_roothash
~~~~~~~~~~~~~~~~~
This property can be utilized for authorization or revocation of
specific dm-verity volumes, identified via their root hashes. It has a
dependency on the DM_VERITY module. This property is controlled by
the ``IPE_PROP_DM_VERITY`` config option, it will be automatically
selected when ``SECURITY_IPE`` and ``DM_VERITY`` are all enabled.
The format of this property is::
dmverity_roothash=DigestName:HexadecimalString
The supported DigestNames for dmverity_roothash are [#dmveritydigests]_
+ blake2b-512
+ blake2s-256
+ sha256
+ sha384
+ sha512
+ sha3-224
+ sha3-256
+ sha3-384
+ sha3-512
+ sm3
+ rmd160
dmverity_signature
~~~~~~~~~~~~~~~~~~
This property can be utilized for authorization of all dm-verity
volumes that have a signed roothash that validated by a keyring
specified by dm-verity's configuration, either the system trusted
keyring, or the secondary keyring. It depends on
``DM_VERITY_VERIFY_ROOTHASH_SIG`` config option and is controlled by
the ``IPE_PROP_DM_VERITY_SIGNATURE`` config option, it will be automatically
selected when ``SECURITY_IPE``, ``DM_VERITY`` and
``DM_VERITY_VERIFY_ROOTHASH_SIG`` are all enabled.
The format of this property is::
dmverity_signature=(TRUE|FALSE)
fsverity_digest
~~~~~~~~~~~~~~~
This property can be utilized for authorization of specific fsverity
enabled files, identified via their fsverity digests.
It depends on ``FS_VERITY`` config option and is controlled by
the ``IPE_PROP_FS_VERITY`` config option, it will be automatically
selected when ``SECURITY_IPE`` and ``FS_VERITY`` are all enabled.
The format of this property is::
fsverity_digest=DigestName:HexadecimalString
The supported DigestNames for fsverity_digest are [#fsveritydigest]_
+ sha256
+ sha512
fsverity_signature
~~~~~~~~~~~~~~~~~~
This property is used to authorize all fs-verity enabled files that have
been verified by fs-verity's built-in signature mechanism. The signature
verification relies on a key stored within the ".fs-verity" keyring. It
depends on ``FS_VERITY_BUILTIN_SIGNATURES`` config option and
it is controlled by the ``IPE_PROP_FS_VERITY`` config option,
it will be automatically selected when ``SECURITY_IPE``, ``FS_VERITY``
and ``FS_VERITY_BUILTIN_SIGNATURES`` are all enabled.
The format of this property is::
fsverity_signature=(TRUE|FALSE)
Policy Examples
---------------
Allow all
~~~~~~~~~
::
policy_name=Allow_All policy_version=0.0.0
DEFAULT action=ALLOW
Allow only initramfs
~~~~~~~~~~~~~~~~~~~~
::
policy_name=Allow_Initramfs policy_version=0.0.0
DEFAULT action=DENY
op=EXECUTE boot_verified=TRUE action=ALLOW
Allow any signed and validated dm-verity volume and the initramfs
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
::
policy_name=Allow_Signed_DMV_And_Initramfs policy_version=0.0.0
DEFAULT action=DENY
op=EXECUTE boot_verified=TRUE action=ALLOW
op=EXECUTE dmverity_signature=TRUE action=ALLOW
Prohibit execution from a specific dm-verity volume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
::
policy_name=Deny_DMV_By_Roothash policy_version=0.0.0
DEFAULT action=DENY
op=EXECUTE dmverity_roothash=sha256:cd2c5bae7c6c579edaae4353049d58eb5f2e8be0244bf05345bc8e5ed257baff action=DENY
op=EXECUTE boot_verified=TRUE action=ALLOW
op=EXECUTE dmverity_signature=TRUE action=ALLOW
Allow only a specific dm-verity volume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
::
policy_name=Allow_DMV_By_Roothash policy_version=0.0.0
DEFAULT action=DENY
op=EXECUTE dmverity_roothash=sha256:401fcec5944823ae12f62726e8184407a5fa9599783f030dec146938 action=ALLOW
Allow any fs-verity file with a valid built-in signature
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
::
policy_name=Allow_Signed_And_Validated_FSVerity policy_version=0.0.0
DEFAULT action=DENY
op=EXECUTE fsverity_signature=TRUE action=ALLOW
Allow execution of a specific fs-verity file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
::
policy_name=ALLOW_FSV_By_Digest policy_version=0.0.0
DEFAULT action=DENY
op=EXECUTE fsverity_digest=sha256:fd88f2b8824e197f850bf4c5109bea5cf0ee38104f710843bb72da796ba5af9e action=ALLOW
Additional Information
----------------------
- `Github Repository <https://github.com/microsoft/ipe>`_
- :doc:`Developer and design docs for IPE </security/ipe>`
FAQ
---
Q:
What's the difference between other LSMs which provide a measure of
trust-based access control?
A:
In general, there's two other LSMs that can provide similar functionality:
IMA, and Loadpin.
IMA and IPE are functionally very similar. The significant difference between
the two is the policy. [#devdoc]_
Loadpin and IPE differ fairly dramatically, as Loadpin only covers the IPE's
kernel read operations, whereas IPE is capable of controlling execution
on top of kernel read. The trust model is also different; Loadpin roots its
trust in the initial super-block, whereas trust in IPE is stemmed from kernel
itself (via ``SYSTEM_TRUSTED_KEYS``).
-----------
.. [#digest_cache_lsm] https://lore.kernel.org/lkml/20240415142436.2545003-1-roberto.sassu@huaweicloud.com/
.. [#interpreters] There is `some interest in solving this issue <https://lore.kernel.org/lkml/20220321161557.495388-1-mic@digikod.net/>`_.
.. [#devdoc] Please see :doc:`the design docs </security/ipe>` for more on
this topic.
.. [#switch_root] https://man7.org/linux/man-pages/man8/switch_root.8.html
.. [#dmveritydigests] These hash algorithms are based on values accepted by
the Linux crypto API; IPE does not impose any
restrictions on the digest algorithm itself;
thus, this list may be out of date.
.. [#fsveritydigest] These hash algorithms are based on values accepted by the
kernel's fsverity support; IPE does not impose any
restrictions on the digest algorithm itself;
thus, this list may be out of date.

12
Documentation/admin-guide/kernel-parameters.txt
View File

@ -2350,6 +2350,18 @@
ipcmni_extend [KNL,EARLY] Extend the maximum number of unique System V
IPC identifiers from 32,768 to 16,777,216.
ipe.enforce= [IPE]
Format: <bool>
Determine whether IPE starts in permissive (0) or
enforce (1) mode. The default is enforce.
ipe.success_audit=
[IPE]
Format: <bool>
Start IPE with success auditing enabled, emitting
an audit event when a binary is allowed. The default
is 0.
irqaffinity= [SMP] Set the default irq affinity mask
The argument is a cpu list, as described above.

6
Documentation/filesystems/fsverity.rst
View File

@ -92,7 +92,9 @@ authenticating fs-verity file hashes include:
"IPE policy" specifically allows for the authorization of fs-verity
files using properties ``fsverity_digest`` for identifying
files by their verity digest, and ``fsverity_signature`` to authorize
files with a verified fs-verity's built-in signature.
files with a verified fs-verity's built-in signature. For
details on configuring IPE policies and understanding its operational
modes, please refer to :doc:`IPE admin guide </admin-guide/LSM/ipe>`.
- Trusted userspace code in combination with `Built-in signature
verification`_. This approach should be used only with great care.
@ -508,6 +510,8 @@ be carefully considered before using them:
files with a verified fs-verity builtin signature to perform certain
operations, such as execution. Note that IPE doesn't require
fs.verity.require_signatures=1.
Please refer to :doc:`IPE admin guide </admin-guide/LSM/ipe>` for
more details.
- A file's builtin signature can only be set at the same time that
fs-verity is being enabled on the file. Changing or deleting the

1
Documentation/security/index.rst
View File

@ -19,3 +19,4 @@ Security Documentation
digsig
landlock
secrets/index
ipe

446
Documentation/security/ipe.rst Normal file
View File

@ -0,0 +1,446 @@
.. SPDX-License-Identifier: GPL-2.0
Integrity Policy Enforcement (IPE) - Kernel Documentation
=========================================================
.. NOTE::
This is documentation targeted at developers, instead of administrators.
If you're looking for documentation on the usage of IPE, please see
:doc:`IPE admin guide </admin-guide/LSM/ipe>`.
Historical Motivation
---------------------
The original issue that prompted IPE's implementation was the creation
of a locked-down system. This system would be born-secure, and have
strong integrity guarantees over both the executable code, and specific
*data files* on the system, that were critical to its function. These
specific data files would not be readable unless they passed integrity
policy. A mandatory access control system would be present, and
as a result, xattrs would have to be protected. This lead to a selection
of what would provide the integrity claims. At the time, there were two
main mechanisms considered that could guarantee integrity for the system
with these requirements:
1. IMA + EVM Signatures
2. DM-Verity
Both options were carefully considered, however the choice to use DM-Verity
over IMA+EVM as the *integrity mechanism* in the original use case of IPE
was due to three main reasons:
1. Protection of additional attack vectors:
* With IMA+EVM, without an encryption solution, the system is vulnerable
to offline attack against the aforementioned specific data files.
Unlike executables, read operations (like those on the protected data
files), cannot be enforced to be globally integrity verified. This means
there must be some form of selector to determine whether a read should
enforce the integrity policy, or it should not.
At the time, this was done with mandatory access control labels. An IMA
policy would indicate what labels required integrity verification, which
presented an issue: EVM would protect the label, but if an attacker could
modify filesystem offline, the attacker could wipe all the xattrs -
including the SELinux labels that would be used to determine whether the
file should be subject to integrity policy.
With DM-Verity, as the xattrs are saved as part of the Merkel tree, if
offline mount occurs against the filesystem protected by dm-verity, the
checksum no longer matches and the file fails to be read.
* As userspace binaries are paged in Linux, dm-verity also offers the
additional protection against a hostile block device. In such an attack,
the block device reports the appropriate content for the IMA hash
initially, passing the required integrity check. Then, on the page fault
that accesses the real data, will report the attacker's payload. Since
dm-verity will check the data when the page fault occurs (and the disk
access), this attack is mitigated.
2. Performance:
* dm-verity provides integrity verification on demand as blocks are
read versus requiring the entire file being read into memory for
validation.
3. Simplicity of signing:
* No need for two signatures (IMA, then EVM): one signature covers
an entire block device.
* Signatures can be stored externally to the filesystem metadata.
* The signature supports an x.509-based signing infrastructure.
The next step was to choose a *policy* to enforce the integrity mechanism.
The minimum requirements for the policy were:
1. The policy itself must be integrity verified (preventing trivial
attack against it).
2. The policy itself must be resistant to rollback attacks.
3. The policy enforcement must have a permissive-like mode.
4. The policy must be able to be updated, in its entirety, without
a reboot.
5. Policy updates must be atomic.
6. The policy must support *revocations* of previously authored
components.
7. The policy must be auditable, at any point-of-time.
IMA, as the only integrity policy mechanism at the time, was
considered against these list of requirements, and did not fulfill
all of the minimum requirements. Extending IMA to cover these
requirements was considered, but ultimately discarded for a
two reasons:
1. Regression risk; many of these changes would result in
dramatic code changes to IMA, which is already present in the
kernel, and therefore might impact users.
2. IMA was used in the system for measurement and attestation;
separation of measurement policy from local integrity policy
enforcement was considered favorable.
Due to these reasons, it was decided that a new LSM should be created,
whose responsibility would be only the local integrity policy enforcement.
Role and Scope
--------------
IPE, as its name implies, is fundamentally an integrity policy enforcement
solution; IPE does not mandate how integrity is provided, but instead
leaves that decision to the system administrator to set the security bar,
via the mechanisms that they select that suit their individual needs.
There are several different integrity solutions that provide a different
level of security guarantees; and IPE allows sysadmins to express policy for
theoretically all of them.
IPE does not have an inherent mechanism to ensure integrity on its own.
Instead, there are more effective layers available for building systems that
can guarantee integrity. It's important to note that the mechanism for proving
integrity is independent of the policy for enforcing that integrity claim.
Therefore, IPE was designed around:
1. Easy integrations with integrity providers.
2. Ease of use for platform administrators/sysadmins.
Design Rationale:
-----------------
IPE was designed after evaluating existing integrity policy solutions
in other operating systems and environments. In this survey of other
implementations, there were a few pitfalls identified:
1. Policies were not readable by humans, usually requiring a binary
intermediary format.
2. A single, non-customizable action was implicitly taken as a default.
3. Debugging the policy required manual steps to determine what rule was violated.
4. Authoring a policy required an in-depth knowledge of the larger system,
or operating system.
IPE attempts to avoid all of these pitfalls.
Policy
~~~~~~
Plain Text
^^^^^^^^^^
IPE's policy is plain-text. This introduces slightly larger policy files than
other LSMs, but solves two major problems that occurs with some integrity policy
solutions on other platforms.
The first issue is one of code maintenance and duplication. To author policies,
the policy has to be some form of string representation (be it structured,
through XML, JSON, YAML, etcetera), to allow the policy author to understand
what is being written. In a hypothetical binary policy design, a serializer
is necessary to write the policy from the human readable form, to the binary
form, and a deserializer is needed to interpret the binary form into a data
structure in the kernel.
Eventually, another deserializer will be needed to transform the binary from
back into the human-readable form with as much information preserved. This is because a
user of this access control system will have to keep a lookup table of a checksum
and the original file itself to try to understand what policies have been deployed
on this system and what policies have not. For a single user, this may be alright,
as old policies can be discarded almost immediately after the update takes hold.
For users that manage computer fleets in the thousands, if not hundreds of thousands,
with multiple different operating systems, and multiple different operational needs,
this quickly becomes an issue, as stale policies from years ago may be present,
quickly resulting in the need to recover the policy or fund extensive infrastructure
to track what each policy contains.
With now three separate serializer/deserializers, maintenance becomes costly. If the
policy avoids the binary format, there is only one required serializer: from the
human-readable form to the data structure in kernel, saving on code maintenance,
and retaining operability.
The second issue with a binary format is one of transparency. As IPE controls
access based on the trust of the system's resources, it's policy must also be
trusted to be changed. This is done through signatures, resulting in needing
signing as a process. Signing, as a process, is typically done with a
high security bar, as anything signed can be used to attack integrity
enforcement systems. It is also important that, when signing something, that
the signer is aware of what they are signing. A binary policy can cause
obfuscation of that fact; what signers see is an opaque binary blob. A
plain-text policy, on the other hand, the signers see the actual policy
submitted for signing.
Boot Policy
~~~~~~~~~~~
IPE, if configured appropriately, is able to enforce a policy as soon as a
kernel is booted and usermode starts. That implies some level of storage
of the policy to apply the minute usermode starts. Generally, that storage
can be handled in one of three ways:
1. The policy file(s) live on disk and the kernel loads the policy prior
to an code path that would result in an enforcement decision.
2. The policy file(s) are passed by the bootloader to the kernel, who
parses the policy.
3. There is a policy file that is compiled into the kernel that is
parsed and enforced on initialization.
The first option has problems: the kernel reading files from userspace
is typically discouraged and very uncommon in the kernel.
The second option also has problems: Linux supports a variety of bootloaders
across its entire ecosystem - every bootloader would have to support this
new methodology or there must be an independent source. It would likely
result in more drastic changes to the kernel startup than necessary.
The third option is the best but it's important to be aware that the policy
will take disk space against the kernel it's compiled in. It's important to
keep this policy generalized enough that userspace can load a new, more
complicated policy, but restrictive enough that it will not overauthorize
and cause security issues.
The initramfs provides a way that this bootup path can be established. The
kernel starts with a minimal policy, that trusts the initramfs only. Inside
the initramfs, when the real rootfs is mounted, but not yet transferred to,
it deploys and activates a policy that trusts the new root filesystem.
This prevents overauthorization at any step, and keeps the kernel policy
to a minimal size.
Startup
^^^^^^^
Not every system, however starts with an initramfs, so the startup policy
compiled into the kernel will need some flexibility to express how trust
is established for the next phase of the bootup. To this end, if we just
make the compiled-in policy a full IPE policy, it allows system builders
to express the first stage bootup requirements appropriately.
Updatable, Rebootless Policy
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
As requirements change over time (vulnerabilities are found in previously
trusted applications, keys roll, etcetera). Updating a kernel to change the
meet those security goals is not always a suitable option, as updates are not
always risk-free, and blocking a security update leaves systems vulnerable.
This means IPE requires a policy that can be completely updated (allowing
revocations of existing policy) from a source external to the kernel (allowing
policies to be updated without updating the kernel).
Additionally, since the kernel is stateless between invocations, and reading
policy files off the disk from kernel space is a bad idea(tm), then the
policy updates have to be done rebootlessly.
To allow an update from an external source, it could be potentially malicious,
so this policy needs to have a way to be identified as trusted. This is
done via a signature chained to a trust source in the kernel. Arbitrarily,
this is the ``SYSTEM_TRUSTED_KEYRING``, a keyring that is initially
populated at kernel compile-time, as this matches the expectation that the
author of the compiled-in policy described above is the same entity that can
deploy policy updates.
Anti-Rollback / Anti-Replay
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Over time, vulnerabilities are found and trusted resources may not be
trusted anymore. IPE's policy has no exception to this. There can be
instances where a mistaken policy author deploys an insecure policy,
before correcting it with a secure policy.
Assuming that as soon as the insecure policy is signed, and an attacker
acquires the insecure policy, IPE needs a way to prevent rollback
from the secure policy update to the insecure policy update.
Initially, IPE's policy can have a policy_version that states the
minimum required version across all policies that can be active on
the system. This will prevent rollback while the system is live.
.. WARNING::
However, since the kernel is stateless across boots, this policy
version will be reset to 0.0.0 on the next boot. System builders
need to be aware of this, and ensure the new secure policies are
deployed ASAP after a boot to ensure that the window of
opportunity is minimal for an attacker to deploy the insecure policy.
Implicit Actions:
~~~~~~~~~~~~~~~~~
The issue of implicit actions only becomes visible when you consider
a mixed level of security bars across multiple operations in a system.
For example, consider a system that has strong integrity guarantees
over both the executable code, and specific *data files* on the system,
that were critical to its function. In this system, three types of policies
are possible:
1. A policy in which failure to match any rules in the policy results
in the action being denied.
2. A policy in which failure to match any rules in the policy results
in the action being allowed.
3. A policy in which the action taken when no rules are matched is
specified by the policy author.
The first option could make a policy like this::
op=EXECUTE integrity_verified=YES action=ALLOW
In the example system, this works well for the executables, as all
executables should have integrity guarantees, without exception. The
issue becomes with the second requirement about specific data files.
This would result in a policy like this (assuming each line is
evaluated in order)::
op=EXECUTE integrity_verified=YES action=ALLOW
op=READ integrity_verified=NO label=critical_t action=DENY
op=READ action=ALLOW
This is somewhat clear if you read the docs, understand the policy
is executed in order and that the default is a denial; however, the
last line effectively changes that default to an ALLOW. This is
required, because in a realistic system, there are some unverified
reads (imagine appending to a log file).
The second option, matching no rules results in an allow, is clearer
for the specific data files::
op=READ integrity_verified=NO label=critical_t action=DENY
And, like the first option, falls short with the execution scenario,
effectively needing to override the default::
op=EXECUTE integrity_verified=YES action=ALLOW
op=EXECUTE action=DENY
op=READ integrity_verified=NO label=critical_t action=DENY
This leaves the third option. Instead of making users be clever
and override the default with an empty rule, force the end-user
to consider what the appropriate default should be for their
scenario and explicitly state it::
DEFAULT op=EXECUTE action=DENY
op=EXECUTE integrity_verified=YES action=ALLOW
DEFAULT op=READ action=ALLOW
op=READ integrity_verified=NO label=critical_t action=DENY
Policy Debugging:
~~~~~~~~~~~~~~~~~
When developing a policy, it is useful to know what line of the policy
is being violated to reduce debugging costs; narrowing the scope of the
investigation to the exact line that resulted in the action. Some integrity
policy systems do not provide this information, instead providing the
information that was used in the evaluation. This then requires a correlation
with the policy to evaluate what went wrong.
Instead, IPE just emits the rule that was matched. This limits the scope
of the investigation to the exact policy line (in the case of a specific
rule), or the section (in the case of a DEFAULT). This decreases iteration
and investigation times when policy failures are observed while evaluating
policies.
IPE's policy engine is also designed in a way that it makes it obvious to
a human of how to investigate a policy failure. Each line is evaluated in
the sequence that is written, so the algorithm is very simple to follow
for humans to recreate the steps and could have caused the failure. In other
surveyed systems, optimizations occur (sorting rules, for instance) when loading
the policy. In those systems, it requires multiple steps to debug, and the
algorithm may not always be clear to the end-user without reading the code first.
Simplified Policy:
~~~~~~~~~~~~~~~~~~
Finally, IPE's policy is designed for sysadmins, not kernel developers. Instead
of covering individual LSM hooks (or syscalls), IPE covers operations. This means
instead of sysadmins needing to know that the syscalls ``mmap``, ``mprotect``,
``execve``, and ``uselib`` must have rules protecting them, they must simple know
that they want to restrict code execution. This limits the amount of bypasses that
could occur due to a lack of knowledge of the underlying system; whereas the
maintainers of IPE, being kernel developers can make the correct choice to determine
whether something maps to these operations, and under what conditions.
Implementation Notes
--------------------
Anonymous Memory
~~~~~~~~~~~~~~~~
Anonymous memory isn't treated any differently from any other access in IPE.
When anonymous memory is mapped with ``+X``, it still comes into the ``file_mmap``
or ``file_mprotect`` hook, but with a ``NULL`` file object. This is submitted to
the evaluation, like any other file. However, all current trust properties will
evaluate to false, as they are all file-based and the operation is not
associated with a file.
.. WARNING::
This also occurs with the ``kernel_load_data`` hook, when the kernel is
loading data from a userspace buffer that is not backed by a file. In this
scenario all current trust properties will also evaluate to false.
Securityfs Interface
~~~~~~~~~~~~~~~~~~~~
The per-policy securityfs tree is somewhat unique. For example, for
a standard securityfs policy tree::
MyPolicy
|- active
|- delete
|- name
|- pkcs7
|- policy
|- update
|- version
The policy is stored in the ``->i_private`` data of the MyPolicy inode.
Tests
-----
IPE has KUnit Tests for the policy parser. Recommended kunitconfig::
CONFIG_KUNIT=y
CONFIG_SECURITY=y
CONFIG_SECURITYFS=y
CONFIG_PKCS7_MESSAGE_PARSER=y
CONFIG_SYSTEM_DATA_VERIFICATION=y
CONFIG_FS_VERITY=y
CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y
CONFIG_BLOCK=y
CONFIG_MD=y
CONFIG_BLK_DEV_DM=y
CONFIG_DM_VERITY=y
CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG=y
CONFIG_NET=y
CONFIG_AUDIT=y
CONFIG_AUDITSYSCALL=y
CONFIG_BLK_DEV_INITRD=y
CONFIG_SECURITY_IPE=y
CONFIG_IPE_PROP_DM_VERITY=y
CONFIG_IPE_PROP_DM_VERITY_SIGNATURE=y
CONFIG_IPE_PROP_FS_VERITY=y
CONFIG_IPE_PROP_FS_VERITY_BUILTIN_SIG=y
CONFIG_SECURITY_IPE_KUNIT_TEST=y
In addition, IPE has a python based integration
`test suite <https://github.com/microsoft/ipe/tree/test-suite>`_ that
can test both user interfaces and enforcement functionalities.