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9876cfe8ec
This sysctl has the very unusual behaviour of not allowing any user (even
CAP_SYS_ADMIN) to reduce the restriction setting, meaning that if you were
to set this sysctl to a more restrictive option in the host pidns you
would need to reboot your machine in order to reset it.
The justification given in [1] is that this is a security feature and thus
it should not be possible to disable. Aside from the fact that we have
plenty of security-related sysctls that can be disabled after being
enabled (fs.protected_symlinks for instance), the protection provided by
the sysctl is to stop users from being able to create a binary and then
execute it. A user with CAP_SYS_ADMIN can trivially do this without
memfd_create(2):
% cat mount-memfd.c
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/mount.h>
#define SHELLCODE "#!/bin/echo this file was executed from this totally private tmpfs:"
int main(void)
{
int fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
assert(fsfd >= 0);
assert(!fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 2));
int dfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
assert(dfd >= 0);
int execfd = openat(dfd, "exe", O_CREAT | O_RDWR | O_CLOEXEC, 0782);
assert(execfd >= 0);
assert(write(execfd, SHELLCODE, strlen(SHELLCODE)) == strlen(SHELLCODE));
assert(!close(execfd));
char *execpath = NULL;
char *argv[] = { "bad-exe", NULL }, *envp[] = { NULL };
execfd = openat(dfd, "exe", O_PATH | O_CLOEXEC);
assert(execfd >= 0);
assert(asprintf(&execpath, "/proc/self/fd/%d", execfd) > 0);
assert(!execve(execpath, argv, envp));
}
% ./mount-memfd
this file was executed from this totally private tmpfs: /proc/self/fd/5
%
Given that it is possible for CAP_SYS_ADMIN users to create executable
binaries without memfd_create(2) and without touching the host filesystem
(not to mention the many other things a CAP_SYS_ADMIN process would be
able to do that would be equivalent or worse), it seems strange to cause a
fair amount of headache to admins when there doesn't appear to be an
actual security benefit to blocking this. There appear to be concerns
about confused-deputy-esque attacks[2] but a confused deputy that can
write to arbitrary sysctls is a bigger security issue than executable
memfds.
/* New API */
The primary requirement from the original author appears to be more based
on the need to be able to restrict an entire system in a hierarchical
manner[3], such that child namespaces cannot re-enable executable memfds.
So, implement that behaviour explicitly -- the vm.memfd_noexec scope is
evaluated up the pidns tree to &init_pid_ns and you have the most
restrictive value applied to you. The new lower limit you can set
vm.memfd_noexec is whatever limit applies to your parent.
Note that a pidns will inherit a copy of the parent pidns's effective
vm.memfd_noexec setting at unshare() time. This matches the existing
behaviour, and it also ensures that a pidns will never have its
vm.memfd_noexec setting *lowered* behind its back (but it will be raised
if the parent raises theirs).
/* Backwards Compatibility */
As the previous version of the sysctl didn't allow you to lower the
setting at all, there are no backwards compatibility issues with this
aspect of the change.
However it should be noted that now that the setting is completely
hierarchical. Previously, a cloned pidns would just copy the current
pidns setting, meaning that if the parent's vm.memfd_noexec was changed it
wouldn't propoagate to existing pid namespaces. Now, the restriction
applies recursively. This is a uAPI change, however:
* The sysctl is very new, having been merged in 6.3.
* Several aspects of the sysctl were broken up until this patchset and
the other patchset by Jeff Xu last month.
And thus it seems incredibly unlikely that any real users would run into
this issue. In the worst case, if this causes userspace isues we could
make it so that modifying the setting follows the hierarchical rules but
the restriction checking uses the cached copy.
[1]: https://lore.kernel.org/CABi2SkWnAgHK1i6iqSqPMYuNEhtHBkO8jUuCvmG3RmUB5TKHJw@mail.gmail.com/
[2]: https://lore.kernel.org/CALmYWFs_dNCzw_pW1yRAo4bGCPEtykroEQaowNULp7svwMLjOg@mail.gmail.com/
[3]: https://lore.kernel.org/CALmYWFuahdUF7cT4cm7_TGLqPanuHXJ-hVSfZt7vpTnc18DPrw@mail.gmail.com/
Link: https://lkml.kernel.org/r/20230814-memfd-vm-noexec-uapi-fixes-v2-4-7ff9e3e10ba6@cyphar.com
Fixes: 105ff5339f49 ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Cc: Dominique Martinet <asmadeus@codewreck.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Daniel Verkamp <dverkamp@chromium.org>
Cc: Jeff Xu <jeffxu@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
127 lines
3.1 KiB
C
127 lines
3.1 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_PID_NS_H
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#define _LINUX_PID_NS_H
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#include <linux/sched.h>
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#include <linux/bug.h>
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#include <linux/mm.h>
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#include <linux/workqueue.h>
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#include <linux/threads.h>
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#include <linux/nsproxy.h>
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#include <linux/ns_common.h>
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#include <linux/idr.h>
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/* MAX_PID_NS_LEVEL is needed for limiting size of 'struct pid' */
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#define MAX_PID_NS_LEVEL 32
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struct fs_pin;
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#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
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/* modes for vm.memfd_noexec sysctl */
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#define MEMFD_NOEXEC_SCOPE_EXEC 0 /* MFD_EXEC implied if unset */
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#define MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL 1 /* MFD_NOEXEC_SEAL implied if unset */
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#define MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED 2 /* same as 1, except MFD_EXEC rejected */
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#endif
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struct pid_namespace {
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struct idr idr;
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struct rcu_head rcu;
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unsigned int pid_allocated;
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struct task_struct *child_reaper;
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struct kmem_cache *pid_cachep;
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unsigned int level;
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struct pid_namespace *parent;
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#ifdef CONFIG_BSD_PROCESS_ACCT
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struct fs_pin *bacct;
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#endif
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struct user_namespace *user_ns;
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struct ucounts *ucounts;
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int reboot; /* group exit code if this pidns was rebooted */
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struct ns_common ns;
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#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
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int memfd_noexec_scope;
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#endif
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} __randomize_layout;
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extern struct pid_namespace init_pid_ns;
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#define PIDNS_ADDING (1U << 31)
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#ifdef CONFIG_PID_NS
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static inline struct pid_namespace *get_pid_ns(struct pid_namespace *ns)
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{
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if (ns != &init_pid_ns)
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refcount_inc(&ns->ns.count);
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return ns;
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}
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#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
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static inline int pidns_memfd_noexec_scope(struct pid_namespace *ns)
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{
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int scope = MEMFD_NOEXEC_SCOPE_EXEC;
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for (; ns; ns = ns->parent)
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scope = max(scope, READ_ONCE(ns->memfd_noexec_scope));
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return scope;
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}
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#else
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static inline int pidns_memfd_noexec_scope(struct pid_namespace *ns)
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{
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return 0;
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}
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#endif
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extern struct pid_namespace *copy_pid_ns(unsigned long flags,
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struct user_namespace *user_ns, struct pid_namespace *ns);
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extern void zap_pid_ns_processes(struct pid_namespace *pid_ns);
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extern int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd);
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extern void put_pid_ns(struct pid_namespace *ns);
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#else /* !CONFIG_PID_NS */
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#include <linux/err.h>
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static inline struct pid_namespace *get_pid_ns(struct pid_namespace *ns)
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{
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return ns;
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}
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static inline int pidns_memfd_noexec_scope(struct pid_namespace *ns)
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{
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return 0;
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}
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static inline struct pid_namespace *copy_pid_ns(unsigned long flags,
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struct user_namespace *user_ns, struct pid_namespace *ns)
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{
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if (flags & CLONE_NEWPID)
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ns = ERR_PTR(-EINVAL);
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return ns;
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}
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static inline void put_pid_ns(struct pid_namespace *ns)
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{
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}
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static inline void zap_pid_ns_processes(struct pid_namespace *ns)
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{
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BUG();
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}
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static inline int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd)
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{
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return 0;
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}
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#endif /* CONFIG_PID_NS */
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extern struct pid_namespace *task_active_pid_ns(struct task_struct *tsk);
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void pidhash_init(void);
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void pid_idr_init(void);
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static inline bool task_is_in_init_pid_ns(struct task_struct *tsk)
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{
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return task_active_pid_ns(tsk) == &init_pid_ns;
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}
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#endif /* _LINUX_PID_NS_H */
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