CVE-2026-89638: smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions

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

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

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

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

smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions

When a file has the setuid or setgid bit set and is written to, the VFS
strips those bits and issues a setattr with ATTR_KILL_SUID/ATTR_KILL_SGID
together with an ATTR_MODE carrying the already-cleared mode.

Both cifs_setattr_unix() and cifs_setattr_nounix() unconditionally dropped
ATTR_MODE in that case:

	/* skip mode change if it's just for clearing setuid/setgid */
	if (attrs->ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID))
		attrs->ia_valid &= ~ATTR_MODE;

This is fine for the default mount, where the mode is only emulated via
the DOS read-only attribute and cannot represent the setuid/setgid bits
anyway.  However, with the "cifsacl" or "modefromsid" mount options the
mode is stored on the server through an ACL (id_mode_to_cifs_acl()), with
the SMB3.1.1 POSIX extensions the mode is sent to the server directly,
and with the SMB1 Unix extensions (cifs_setattr_unix) the mode is sent
via CIFSSMBUnixSetPathInfo().  In all those cases dropping ATTR_MODE means
the cleared mode is never pushed to the server, so the setuid/setgid bit
survives the write.

This is a security issue: on local filesystems the setuid bit is stripped
when a file is written, but over these cifs.ko mounts the bit persists on
the server, potentially allowing an unexpected privilege escalation on
subsequent execution.

Fix this in two places:

  1. cifs_setattr_nounix(): only take the "skip mode change" shortcut
     when the mode is emulated via the DOS read-only attribute (i.e.
     neither cifsacl/modefromsid nor the SMB3.1.1 POSIX extensions are
     in effect), so that the cleared mode is propagated to the server
     in the ACL / POSIX cases.

  2. cifs_setattr_unix(): this function is only called when Unix
     extensions are in effect, so the mode is always stored on the
     server.  Remove the shortcut entirely so that the cleared mode is
     always pushed.

The Linux kernel CVE team has assigned CVE-2026-89638 to this issue.


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

	Issue introduced in 2.6.24 with commit d32c4f2626acc385d5187bd5c0c24f442328dc60 and fixed in 6.18.51 with commit b10015807e4c628095d1d1d1c9307efc8cdd9e1b
	Issue introduced in 2.6.24 with commit d32c4f2626acc385d5187bd5c0c24f442328dc60 and fixed in 7.2.4 with commit 69bfe810ecd1e0387d8975d711cd326341d13c6e
	Issue introduced in 2.6.24 with commit d32c4f2626acc385d5187bd5c0c24f442328dc60 and fixed in 7.3-rc1 with commit b8e5dc4f95e5484159b343903f302eb6d783f2e6

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-2026-89638
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/inode.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/b10015807e4c628095d1d1d1c9307efc8cdd9e1b
	https://git.kernel.org/stable/c/69bfe810ecd1e0387d8975d711cd326341d13c6e
	https://git.kernel.org/stable/c/b8e5dc4f95e5484159b343903f302eb6d783f2e6