CVE-2026-89477: sctp: fix NULL deref on untransmitted RECONF completion

CVE-2026-89477: sctp: fix NULL deref on untransmitted RECONF completion

[Greg Kroah-Hartman]

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

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

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

sctp: fix NULL deref on untransmitted RECONF completion

sctp_process_strreset_outreq(), sctp_process_strreset_addstrm_out() and
sctp_process_strreset_resp() complete a pending stream reconfiguration
request by stopping the reconf timer on the transport it was sent on:

	t = asoc->strreset_chunk->transport;
	if (timer_delete(&t->reconf_timer))
		sctp_transport_put(t);

chunk->transport is assigned by __sctp_packet_append_chunk() when the
chunk is appended to an outbound packet, and sctp_outq_flush_ctrl() arms
the reconf timer at that same point. A request already published in
asoc->strreset_chunk but not yet transmitted has neither, so completing
it dereferences NULL.

Two ways to get there. sctp_send_asconf_del_ip() sets
asoc->src_out_of_asoc_ok without sending anything when the address being
removed is the association's last one, and sctp_outq_flush_ctrl() then
leaves every non-ASCONF control chunk queued; as only
sctp_process_asconf_ack() clears that flag, it persists. An unprivileged
process that removes such an address and then asks for a stream reset
panics the kernel from softirq. A peer needs neither ASCONF nor local
help: sctp_cmd_interpreter() uncorks the outqueue only once the whole
packet has been processed, so a reply built while walking a RECONF chunk
stays untransmitted for the rest of that walk, and one RECONF chunk
carrying [Incoming SSN Reset Request, Outgoing SSN Reset Request,
Response] -- or two RECONF chunks in one packet -- reaches the same
dereference.

  KASAN: null-ptr-deref in range [0x00000000000001e8-0x00000000000001ef]
  RIP: 0010:timer_delete+0x67/0x110
  Call Trace:
   <IRQ>
   sctp_process_strreset_addstrm_out (net/sctp/stream.c:832)
   sctp_sf_do_reconf (net/sctp/sm_statefuns.c:4212)
   sctp_do_sm (net/sctp/sm_sideeffect.c:1172)
   sctp_assoc_bh_rcv (net/sctp/associola.c:1044)
   sctp_rcv (net/sctp/input.c:243)
   ip_local_deliver (net/ipv4/ip_input.c:262)
   process_backlog (net/core/dev.c:6680)
   </IRQ>

A response can only acknowledge a request that was actually sent, so do
not match asoc->strreset_chunk while chunk->transport is NULL. Guarding
the lookup covers all three completion sites.

The Linux kernel CVE team has assigned CVE-2026-89477 to this issue.


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

	Issue introduced in 4.11 with commit 81054476453640159149cb6704e834893e16736b and fixed in 6.12.109 with commit 77ea60f2d24002c308f224db44a4542c466489af
	Issue introduced in 4.11 with commit 81054476453640159149cb6704e834893e16736b and fixed in 6.18.50 with commit 25419f516ea84b1e7d3d55aae5215ac86ece0c66
	Issue introduced in 4.11 with commit 81054476453640159149cb6704e834893e16736b and fixed in 7.2.4 with commit 0ee697defc0594f1da810ed75d9d643fb46f5318
	Issue introduced in 4.11 with commit 81054476453640159149cb6704e834893e16736b and fixed in 7.3-rc1 with commit 2db9bfa3e27bdea15e05ea70b56bad3d21e570ec

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-2026-89477
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/stream.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/77ea60f2d24002c308f224db44a4542c466489af
	https://git.kernel.org/stable/c/25419f516ea84b1e7d3d55aae5215ac86ece0c66
	https://git.kernel.org/stable/c/0ee697defc0594f1da810ed75d9d643fb46f5318
	https://git.kernel.org/stable/c/2db9bfa3e27bdea15e05ea70b56bad3d21e570ec