linux-next/Documentation/security/SCTP.rst

.. SPDX-License-Identifier: GPL-2.0

====
SCTP
====

SCTP LSM Support
================

Security Hooks
--------------

For security module support, three SCTP specific hooks have been implemented::

    security_sctp_assoc_request()
    security_sctp_bind_connect()
    security_sctp_sk_clone()
    security_sctp_assoc_established()

The usage of these hooks are described below with the SELinux implementation
described in the `SCTP SELinux Support`_ chapter.


security_sctp_assoc_request()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Passes the ``@asoc`` and ``@chunk->skb`` of the association INIT packet to the
security module. Returns 0 on success, error on failure.
::

    @asoc - pointer to sctp association structure.
    @skb - pointer to skbuff of association packet.


security_sctp_bind_connect()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Passes one or more ipv4/ipv6 addresses to the security module for validation
based on the ``@optname`` that will result in either a bind or connect
service as shown in the permission check tables below.
Returns 0 on success, error on failure.
::

    @sk      - Pointer to sock structure.
    @optname - Name of the option to validate.
    @address - One or more ipv4 / ipv6 addresses.
    @addrlen - The total length of address(s). This is calculated on each
               ipv4 or ipv6 address using sizeof(struct sockaddr_in) or
               sizeof(struct sockaddr_in6).

  ------------------------------------------------------------------
  |                     BIND Type Checks                           |
  |       @optname             |         @address contains         |
  |----------------------------|-----------------------------------|
  | SCTP_SOCKOPT_BINDX_ADD     | One or more ipv4 / ipv6 addresses |
  | SCTP_PRIMARY_ADDR          | Single ipv4 or ipv6 address       |
  | SCTP_SET_PEER_PRIMARY_ADDR | Single ipv4 or ipv6 address       |
  ------------------------------------------------------------------

  ------------------------------------------------------------------
  |                   CONNECT Type Checks                          |
  |       @optname             |         @address contains         |
  |----------------------------|-----------------------------------|
  | SCTP_SOCKOPT_CONNECTX      | One or more ipv4 / ipv6 addresses |
  | SCTP_PARAM_ADD_IP          | One or more ipv4 / ipv6 addresses |
  | SCTP_SENDMSG_CONNECT       | Single ipv4 or ipv6 address       |
  | SCTP_PARAM_SET_PRIMARY     | Single ipv4 or ipv6 address       |
  ------------------------------------------------------------------

A summary of the ``@optname`` entries is as follows::

    SCTP_SOCKOPT_BINDX_ADD - Allows additional bind addresses to be
                             associated after (optionally) calling
                             bind(3).
                             sctp_bindx(3) adds a set of bind
                             addresses on a socket.

    SCTP_SOCKOPT_CONNECTX - Allows the allocation of multiple
                            addresses for reaching a peer
                            (multi-homed).
                            sctp_connectx(3) initiates a connection
                            on an SCTP socket using multiple
                            destination addresses.

    SCTP_SENDMSG_CONNECT  - Initiate a connection that is generated by a
                            sendmsg(2) or sctp_sendmsg(3) on a new association.

    SCTP_PRIMARY_ADDR     - Set local primary address.

    SCTP_SET_PEER_PRIMARY_ADDR - Request peer sets address as
                                 association primary.

    SCTP_PARAM_ADD_IP          - These are used when Dynamic Address
    SCTP_PARAM_SET_PRIMARY     - Reconfiguration is enabled as explained below.


To support Dynamic Address Reconfiguration the following parameters must be
enabled on both endpoints (or use the appropriate **setsockopt**\(2))::

    /proc/sys/net/sctp/addip_enable
    /proc/sys/net/sctp/addip_noauth_enable

then the following *_PARAM_*'s are sent to the peer in an
ASCONF chunk when the corresponding ``@optname``'s are present::

          @optname                      ASCONF Parameter
         ----------                    ------------------
    SCTP_SOCKOPT_BINDX_ADD     ->   SCTP_PARAM_ADD_IP
    SCTP_SET_PEER_PRIMARY_ADDR ->   SCTP_PARAM_SET_PRIMARY


security_sctp_sk_clone()
~~~~~~~~~~~~~~~~~~~~~~~~
Called whenever a new socket is created by **accept**\(2)
(i.e. a TCP style socket) or when a socket is 'peeled off' e.g userspace
calls **sctp_peeloff**\(3).
::

    @asoc - pointer to current sctp association structure.
    @sk - pointer to current sock structure.
    @newsk - pointer to new sock structure.


security_sctp_assoc_established()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Called when a COOKIE ACK is received, and the peer secid will be
saved into ``@asoc->peer_secid`` for client::

    @asoc - pointer to sctp association structure.
    @skb - pointer to skbuff of the COOKIE ACK packet.


Security Hooks used for Association Establishment
-------------------------------------------------

The following diagram shows the use of ``security_sctp_bind_connect()``,
``security_sctp_assoc_request()``, ``security_sctp_assoc_established()`` when
establishing an association.
::

      SCTP endpoint "A"                                SCTP endpoint "Z"
      =================                                =================
    sctp_sf_do_prm_asoc()
 Association setup can be initiated
 by a connect(2), sctp_connectx(3),
 sendmsg(2) or sctp_sendmsg(3).
 These will result in a call to
 security_sctp_bind_connect() to
 initiate an association to
 SCTP peer endpoint "Z".
         INIT --------------------------------------------->
                                                   sctp_sf_do_5_1B_init()
                                                 Respond to an INIT chunk.
                                             SCTP peer endpoint "A" is asking
                                             for a temporary association.
                                             Call security_sctp_assoc_request()
                                             to set the peer label if first
                                             association.
                                             If not first association, check
                                             whether allowed, IF so send:
          <----------------------------------------------- INIT ACK
          |                                  ELSE audit event and silently
          |                                       discard the packet.
          |
    COOKIE ECHO ------------------------------------------>
                                                  sctp_sf_do_5_1D_ce()
                                             Respond to an COOKIE ECHO chunk.
                                             Confirm the cookie and create a
                                             permanent association.
                                             Call security_sctp_assoc_request() to
                                             do the same as for INIT chunk Response.
          <------------------------------------------- COOKIE ACK
          |                                               |
    sctp_sf_do_5_1E_ca                                    |
 Call security_sctp_assoc_established()                   |
 to set the peer label.                                   |
          |                                               |
          |                               If SCTP_SOCKET_TCP or peeled off
          |                               socket security_sctp_sk_clone() is
          |                               called to clone the new socket.
          |                                               |
      ESTABLISHED                                    ESTABLISHED
          |                                               |
    ------------------------------------------------------------------
    |                     Association Established                    |
    ------------------------------------------------------------------


SCTP SELinux Support
====================

Security Hooks
--------------

The `SCTP LSM Support`_ chapter above describes the following SCTP security
hooks with the SELinux specifics expanded below::

    security_sctp_assoc_request()
    security_sctp_bind_connect()
    security_sctp_sk_clone()
    security_sctp_assoc_established()


security_sctp_assoc_request()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Passes the ``@asoc`` and ``@chunk->skb`` of the association INIT packet to the
security module. Returns 0 on success, error on failure.
::

    @asoc - pointer to sctp association structure.
    @skb - pointer to skbuff of association packet.

The security module performs the following operations:
     IF this is the first association on ``@asoc->base.sk``, then set the peer
     sid to that in ``@skb``. This will ensure there is only one peer sid
     assigned to ``@asoc->base.sk`` that may support multiple associations.

     ELSE validate the ``@asoc->base.sk peer_sid`` against the ``@skb peer sid``
     to determine whether the association should be allowed or denied.

     Set the sctp ``@asoc sid`` to socket's sid (from ``asoc->base.sk``) with
     MLS portion taken from ``@skb peer sid``. This will be used by SCTP
     TCP style sockets and peeled off connections as they cause a new socket
     to be generated.

     If IP security options are configured (CIPSO/CALIPSO), then the ip
     options are set on the socket.


security_sctp_bind_connect()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Checks permissions required for ipv4/ipv6 addresses based on the ``@optname``
as follows::

  ------------------------------------------------------------------
  |                   BIND Permission Checks                       |
  |       @optname             |         @address contains         |
  |----------------------------|-----------------------------------|
  | SCTP_SOCKOPT_BINDX_ADD     | One or more ipv4 / ipv6 addresses |
  | SCTP_PRIMARY_ADDR          | Single ipv4 or ipv6 address       |
  | SCTP_SET_PEER_PRIMARY_ADDR | Single ipv4 or ipv6 address       |
  ------------------------------------------------------------------

  ------------------------------------------------------------------
  |                 CONNECT Permission Checks                      |
  |       @optname             |         @address contains         |
  |----------------------------|-----------------------------------|
  | SCTP_SOCKOPT_CONNECTX      | One or more ipv4 / ipv6 addresses |
  | SCTP_PARAM_ADD_IP          | One or more ipv4 / ipv6 addresses |
  | SCTP_SENDMSG_CONNECT       | Single ipv4 or ipv6 address       |
  | SCTP_PARAM_SET_PRIMARY     | Single ipv4 or ipv6 address       |
  ------------------------------------------------------------------


`SCTP LSM Support`_ gives a summary of the ``@optname``
entries and also describes ASCONF chunk processing when Dynamic Address
Reconfiguration is enabled.


security_sctp_sk_clone()
~~~~~~~~~~~~~~~~~~~~~~~~
Called whenever a new socket is created by **accept**\(2) (i.e. a TCP style
socket) or when a socket is 'peeled off' e.g userspace calls
**sctp_peeloff**\(3). ``security_sctp_sk_clone()`` will set the new
sockets sid and peer sid to that contained in the ``@asoc sid`` and
``@asoc peer sid`` respectively.
::

    @asoc - pointer to current sctp association structure.
    @sk - pointer to current sock structure.
    @newsk - pointer to new sock structure.


security_sctp_assoc_established()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Called when a COOKIE ACK is received where it sets the connection's peer sid
to that in ``@skb``::

    @asoc - pointer to sctp association structure.
    @skb - pointer to skbuff of the COOKIE ACK packet.


Policy Statements
-----------------
The following class and permissions to support SCTP are available within the
kernel::

    class sctp_socket inherits socket { node_bind }

whenever the following policy capability is enabled::

    policycap extended_socket_class;

SELinux SCTP support adds the ``name_connect`` permission for connecting
to a specific port type and the ``association`` permission that is explained
in the section below.

If userspace tools have been updated, SCTP will support the ``portcon``
statement as shown in the following example::

    portcon sctp 1024-1036 system_u:object_r:sctp_ports_t:s0


SCTP Peer Labeling
------------------
An SCTP socket will only have one peer label assigned to it. This will be
assigned during the establishment of the first association. Any further
associations on this socket will have their packet peer label compared to
the sockets peer label, and only if they are different will the
``association`` permission be validated. This is validated by checking the
socket peer sid against the received packets peer sid to determine whether
the association should be allowed or denied.

NOTES:
   1) If peer labeling is not enabled, then the peer context will always be
      ``SECINITSID_UNLABELED`` (``unlabeled_t`` in Reference Policy).

   2) As SCTP can support more than one transport address per endpoint
      (multi-homing) on a single socket, it is possible to configure policy
      and NetLabel to provide different peer labels for each of these. As the
      socket peer label is determined by the first associations transport
      address, it is recommended that all peer labels are consistent.

   3) **getpeercon**\(3) may be used by userspace to retrieve the sockets peer
      context.

   4) While not SCTP specific, be aware when using NetLabel that if a label
      is assigned to a specific interface, and that interface 'goes down',
      then the NetLabel service will remove the entry. Therefore ensure that
      the network startup scripts call **netlabelctl**\(8) to set the required
      label (see **netlabel-config**\(8) helper script for details).

   5) The NetLabel SCTP peer labeling rules apply as discussed in the following
      set of posts tagged "netlabel" at: https://www.paul-moore.com/blog/t.

   6) CIPSO is only supported for IPv4 addressing: ``socket(AF_INET, ...)``
      CALIPSO is only supported for IPv6 addressing: ``socket(AF_INET6, ...)``

      Note the following when testing CIPSO/CALIPSO:
         a) CIPSO will send an ICMP packet if an SCTP packet cannot be
            delivered because of an invalid label.
         b) CALIPSO does not send an ICMP packet, just silently discards it.

   7) IPSEC is not supported as RFC 3554 - sctp/ipsec support has not been
      implemented in userspace (**racoon**\(8) or **ipsec_pluto**\(8)),
      although the kernel supports SCTP/IPSEC.