CVE-2026-89476: sctp: fix stream->outcnt underflow on duplicate RECONF responses

CVE-2026-89476: sctp: fix stream->outcnt underflow on duplicate RECONF responses

[Greg Kroah-Hartman]

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

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

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

sctp: fix stream->outcnt underflow on duplicate RECONF responses

A cached RECONF chunk may contain more than one request parameter.  A
duplicate response can therefore find and process the same ADD_OUT request
again while another parameter is still outstanding, rolling back outcnt
twice and possibly underflowing it.

Track outstanding request types as bits and clear each bit after its first
response.  Later responses for the same request are then ignored.

The Linux kernel CVE team has assigned CVE-2026-89476 to this issue.


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

	Issue introduced in 4.12 with commit 11ae76e67a179f25ae8f772d62a7ddf717b1cdf3 and fixed in 6.12.109 with commit 2d867663c563e8009257b9edf9c8ec75d9bbf19a
	Issue introduced in 4.12 with commit 11ae76e67a179f25ae8f772d62a7ddf717b1cdf3 and fixed in 6.18.50 with commit d02a5794c3deedcb8476d017fcee2b7aaedab2e1
	Issue introduced in 4.12 with commit 11ae76e67a179f25ae8f772d62a7ddf717b1cdf3 and fixed in 7.2.4 with commit 8320cbd81bc21a1ca6ebdf70ad610ebc5ef1bd68
	Issue introduced in 4.12 with commit 11ae76e67a179f25ae8f772d62a7ddf717b1cdf3 and fixed in 7.3-rc1 with commit 3faf13aff243ca9f78d08b1a2956ef5a6fc77b6e

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-2026-89476
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:
	include/net/sctp/structs.h
	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/2d867663c563e8009257b9edf9c8ec75d9bbf19a
	https://git.kernel.org/stable/c/d02a5794c3deedcb8476d017fcee2b7aaedab2e1
	https://git.kernel.org/stable/c/8320cbd81bc21a1ca6ebdf70ad610ebc5ef1bd68
	https://git.kernel.org/stable/c/3faf13aff243ca9f78d08b1a2956ef5a6fc77b6e