CVE-2026-89637: smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2

CVE-2026-89637: smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2

[Greg Kroah-Hartman]

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

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

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

smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2

When a valid primary TRANSACT2 response has been received (mid->resp_buf
set, mid->multiRsp true) and a subsequent secondary response causes
cifs_check_trans2() to return false -- either because the SMB header is
invalid (malformed != 0) or because check2ndT2() rejects the PDU --
handle_mid() overwrites mid->resp_buf with the new buffer (leaking the
primary buffer) and, because mid->multiRsp is set, skips the
server->smallbuf/bigbuf NULL-out.  When the user thread frees
mid->resp_buf, server->smallbuf or server->bigbuf is left dangling; the
demux thread reuses it for the next packet, resulting in a use-after-free.

Combine both early-exit conditions and, when mid->multiRsp is already
set, abort the pending transaction inline: set multiEnd, call
dequeue_mid() with malformed=true, and return true so handle_mid() exits
without touching mid->resp_buf or the server buffer pointers.

The Linux kernel CVE team has assigned CVE-2026-89637 to this issue.


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

	Issue introduced in 3.6 with commit 316cf94a910f6f93d43cc574359d163ccae098a3 and fixed in 6.18.51 with commit 9eed72e9534b10a6d9f8f5146feff3db53aebdba
	Issue introduced in 3.6 with commit 316cf94a910f6f93d43cc574359d163ccae098a3 and fixed in 7.2.4 with commit 5e6533a683f6a851158d9f33fb4ea8f4f25d7f84
	Issue introduced in 3.6 with commit 316cf94a910f6f93d43cc574359d163ccae098a3 and fixed in 7.3-rc1 with commit 730d0bb19507b9e19c2fe5343109ac618e2fbce5

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-2026-89637
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:
	fs/smb/client/smb1transport.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/9eed72e9534b10a6d9f8f5146feff3db53aebdba
	https://git.kernel.org/stable/c/5e6533a683f6a851158d9f33fb4ea8f4f25d7f84
	https://git.kernel.org/stable/c/730d0bb19507b9e19c2fe5343109ac618e2fbce5