CVE-2026-43350: smb: client: require a full NFS mode SID before reading mode bits

CVE-2026-43350: smb: client: require a full NFS mode SID before reading mode bits

[Greg Kroah-Hartman]

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

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

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

smb: client: require a full NFS mode SID before reading mode bits

parse_dacl() treats an ACE SID matching sid_unix_NFS_mode as an NFS
mode SID and reads sid.sub_auth[2] to recover the mode bits.

That assumes the ACE carries three subauthorities, but compare_sids()
only compares min(a, b) subauthorities.  A malicious server can return
an ACE with num_subauth = 2 and sub_auth[] = {88, 3}, which still
matches sid_unix_NFS_mode and then drives the sub_auth[2] read four
bytes past the end of the ACE.

Require num_subauth >= 3 before treating the ACE as an NFS mode SID.
This keeps the fix local to the special-SID mode path without changing
compare_sids() semantics for the rest of cifsacl.

The Linux kernel CVE team has assigned CVE-2026-43350 to this issue.


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

	Issue introduced in 5.4 with commit e2f8fbfb8d09c06decde162090fac3ee220aa280 and fixed in 6.6.136 with commit b53b8e98c23310294fc45fc686db5ee860311896
	Issue introduced in 5.4 with commit e2f8fbfb8d09c06decde162090fac3ee220aa280 and fixed in 6.12.84 with commit c8eef12af1cc73031639ea7cf16e0b10e2536b0b
	Issue introduced in 5.4 with commit e2f8fbfb8d09c06decde162090fac3ee220aa280 and fixed in 6.18.25 with commit 38a69f08ee82c450d3e4168707fff2e317dc3ff7
	Issue introduced in 5.4 with commit e2f8fbfb8d09c06decde162090fac3ee220aa280 and fixed in 7.0.2 with commit f8488c07bea2431ee12a6067d736578064fa46b4
	Issue introduced in 5.4 with commit e2f8fbfb8d09c06decde162090fac3ee220aa280 and fixed in 7.1-rc1 with commit 2757ad3e4b6f9e0fed4c7739594e702abc5cab21

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-2026-43350
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/cifsacl.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/b53b8e98c23310294fc45fc686db5ee860311896
	https://git.kernel.org/stable/c/c8eef12af1cc73031639ea7cf16e0b10e2536b0b
	https://git.kernel.org/stable/c/38a69f08ee82c450d3e4168707fff2e317dc3ff7
	https://git.kernel.org/stable/c/f8488c07bea2431ee12a6067d736578064fa46b4
	https://git.kernel.org/stable/c/2757ad3e4b6f9e0fed4c7739594e702abc5cab21