CVE-2026-74586: sctp: clear new_transport when removing a peer

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

sctp: clear new_transport when removing a peer

sctp_process_asconf_param() stores a newly added peer transport in
asoc->new_transport. After all parameters in the ASCONF chunk have been
processed, sctp_sf_do_asconf() uses this pointer to send a HEARTBEAT to the
new transport.

An authenticated ASCONF from a remote SCTP peer can add a transport and
remove it again with a wildcard DEL-IP parameter in the same chunk. The
wildcard deletion preserves the transport on which the ASCONF arrived, but
removes the newly added transport through
sctp_assoc_del_nonprimary_peers(). The removal does not clear
asoc->new_transport, leaving it pointing to the removed transport.

sctp_sf_do_asconf() then creates a HEARTBEAT whose chunk->transport points
to the removed transport without holding a transport reference. During
local address replacement, src_out_of_asoc_ok keeps this HEARTBEAT on
control_chunk_list. After the transport is freed by RCU, a successful
ASCONF_ACK for the replacement address releases the queued HEARTBEAT and
sctp_outq_select_transport() reads the freed transport's state.

The issue was found during a static audit of SCTP objects. With an
authenticated peer, the reproducer triggered the same KASAN report in 2
of 2 unpatched runs on a KASAN-enabled netdev/main kernel:

  BUG: KASAN: slab-use-after-free in sctp_outq_select_transport
  Read of size 4 at addr ffff88800b9bd95c by task python3/197

  Call Trace:
   sctp_outq_select_transport+0x549/0x8b0 [sctp]
   sctp_outq_flush+0x306/0x2c60 [sctp]
   sctp_transport_immediate_rtx+0xaf/0x260 [sctp]
   sctp_process_asconf_ack+0xa48/0xf70 [sctp]

  Allocated by task 197:
   sctp_transport_new+0x68/0x650 [sctp]
   sctp_assoc_add_peer+0x258/0x12a0 [sctp]
   sctp_process_asconf+0x5e9/0x1090 [sctp]

  Last potentially related work creation:
   __call_rcu_common.constprop.0+0x77/0xb70
   sctp_assoc_del_nonprimary_peers+0x7c/0xd0 [sctp]
   sctp_process_asconf+0xd9c/0x1090 [sctp]

The first invalid access was a four-byte read of transport->state at
net/sctp/outqueue.c:833. The same reproducer completed the full
authenticated ASCONF and local-address replacement sequence with this
change without a KASAN report or oops.

Clear new_transport when its peer is removed, before it can be used to
create the HEARTBEAT.

The Linux kernel CVE team has assigned CVE-2026-74586 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 3.2 with commit 6af29ccc223b0feb6fc6112281c3fa3cdb1afddf and fixed in 5.10.265 with commit c0f973bb5118dd1b146cda3fcc8af6f6057befec
	Issue introduced in 3.2 with commit 6af29ccc223b0feb6fc6112281c3fa3cdb1afddf and fixed in 5.15.216 with commit 3b539b317cd052236fed0350364ff1268996ba46
	Issue introduced in 3.2 with commit 6af29ccc223b0feb6fc6112281c3fa3cdb1afddf and fixed in 6.1.183 with commit db9d8e3b670f841755bc2018f178472dc6064d27
	Issue introduced in 3.2 with commit 6af29ccc223b0feb6fc6112281c3fa3cdb1afddf and fixed in 6.6.152 with commit 31efa656cf6aface26e88f038c14f22ee6ca1500
	Issue introduced in 3.2 with commit 6af29ccc223b0feb6fc6112281c3fa3cdb1afddf and fixed in 6.12.104 with commit 291accf36febce751021888de5f15090f4875b56
	Issue introduced in 3.2 with commit 6af29ccc223b0feb6fc6112281c3fa3cdb1afddf and fixed in 6.18.45 with commit ca33df36aa0143a1d04f57d2086020c12e7eddb7
	Issue introduced in 3.2 with commit 6af29ccc223b0feb6fc6112281c3fa3cdb1afddf and fixed in 7.1.9 with commit 163847552a571bd55094291f4ffcdc1de0f14a7b
	Issue introduced in 3.2 with commit 6af29ccc223b0feb6fc6112281c3fa3cdb1afddf and fixed in 7.2 with commit beb33f8ee1ca83acddb2a5ae80f3d22ec550b4c3

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74586
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/sctp/associola.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/c0f973bb5118dd1b146cda3fcc8af6f6057befec
	https://git.kernel.org/stable/c/3b539b317cd052236fed0350364ff1268996ba46
	https://git.kernel.org/stable/c/db9d8e3b670f841755bc2018f178472dc6064d27
	https://git.kernel.org/stable/c/31efa656cf6aface26e88f038c14f22ee6ca1500
	https://git.kernel.org/stable/c/291accf36febce751021888de5f15090f4875b56
	https://git.kernel.org/stable/c/ca33df36aa0143a1d04f57d2086020c12e7eddb7
	https://git.kernel.org/stable/c/163847552a571bd55094291f4ffcdc1de0f14a7b
	https://git.kernel.org/stable/c/beb33f8ee1ca83acddb2a5ae80f3d22ec550b4c3