CVE-2026-72170: 9p: skip nlink update in cacheless mode to fix WARN_ON

CVE-2026-72170: 9p: skip nlink update in cacheless mode to fix WARN_ON

[Greg Kroah-Hartman]

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

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

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

9p: skip nlink update in cacheless mode to fix WARN_ON

v9fs_dec_count() unconditionally calls drop_nlink() on regular files,
even when the inode's nlink is already zero. In cacheless mode the
client refetches inode metadata from the server (the source of truth)
on every operation, so by the time v9fs_remove() returns, the locally
cached nlink may already reflect the post-unlink value:

  1. Client initiates unlink, server processes it and sets nlink to 0
  2. Client refetches inode metadata (nlink=0) before unlink returns
  3. Client's v9fs_remove() completes successfully
  4. Client calls v9fs_dec_count() which calls drop_nlink() on nlink=0

This race is easily triggered under heavy unlink workloads, such as
stress-ng's unlink stressor, producing the following warning:

  WARNING: fs/inode.c:417 at drop_nlink+0x4c/0xc8
  Call trace:
   drop_nlink+0x4c/0xc8
   v9fs_remove+0x1e0/0x250 [9p]
   v9fs_vfs_unlink+0x20/0x38 [9p]
   vfs_unlink+0x13c/0x258
   ...

In cacheless mode the server is authoritative and the inode is on its
way out, so locally adjusting nlink buys nothing. Skip v9fs_dec_count()
entirely when neither CACHE_META nor CACHE_LOOSE is set, which both
avoids the warning and removes a class of nlink races (two concurrent
unlinkers observing nlink > 0 and both calling drop_nlink()) that an
nlink == 0 guard alone would only narrow rather than close.

The Linux kernel CVE team has assigned CVE-2026-72170 to this issue.


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

	Issue introduced in 4.17 with commit ac89b2ef9b55924bcf922251f043ba73a32d05bb and fixed in 6.6.145 with commit a5a682b016ef5b5384e28f6d652d47a8f8e73d37
	Issue introduced in 4.17 with commit ac89b2ef9b55924bcf922251f043ba73a32d05bb and fixed in 6.12.97 with commit de79c3f3643841b8659a71958df7cf2a66bfd409
	Issue introduced in 4.17 with commit ac89b2ef9b55924bcf922251f043ba73a32d05bb and fixed in 6.18.40 with commit 8d610017c992de705b304d3d727a6e3a86af6149
	Issue introduced in 4.17 with commit ac89b2ef9b55924bcf922251f043ba73a32d05bb and fixed in 7.1.5 with commit 8faccac11e1369adddf5d80f4a45af93f13b2e1a
	Issue introduced in 4.17 with commit ac89b2ef9b55924bcf922251f043ba73a32d05bb and fixed in 7.2-rc1 with commit 574aa0b4799470ac814479f1138d19efe6262255

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-2026-72170
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/9p/vfs_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/a5a682b016ef5b5384e28f6d652d47a8f8e73d37
	https://git.kernel.org/stable/c/de79c3f3643841b8659a71958df7cf2a66bfd409
	https://git.kernel.org/stable/c/8d610017c992de705b304d3d727a6e3a86af6149
	https://git.kernel.org/stable/c/8faccac11e1369adddf5d80f4a45af93f13b2e1a
	https://git.kernel.org/stable/c/574aa0b4799470ac814479f1138d19efe6262255