CVE-2026-89707: nfsd: release path refs on follow_down() error

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

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

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

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

nfsd: release path refs on follow_down() error

nfsd_cross_mnt() initializes a local struct path with mntget() and
dget() before calling follow_down(). On a negative return the error
arm jumps to out without releasing those references:

    err = follow_down(&path, follow_flags);
    if (err < 0)
            goto out;

follow_down() never drops the caller's entry-time refs on any error
sub-case; for example a pre-cross d_manage() failure leaves path
untouched, so the mntget()/dget() taken on entry survive the call.

Every other early-exit arm in nfsd_cross_mnt() (other-namespace
return, IS_ERR(exp2), and the success tail after the swap) already
calls path_put(&path); the err < 0 arm is the lone omission. The
leak inflates mnt_count and d_count on each failed cross-mount,
blocking umount and pinning dentries against the shrinker, and is
reachable by any authenticated NFS client through nfsd_lookup_dentry
or the NFSv4 READDIR encode path.

Fix by calling path_put(&path) before the goto out in the err < 0
arm so the entry-time refs are released on all follow_down() error
returns.

The Linux kernel CVE team has assigned CVE-2026-89707 to this issue.


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

	Issue introduced in 2.6.38 with commit cc53ce53c86924bfe98a12ea20b7465038a08792 and fixed in 6.12.109 with commit 194316df81263519156ebe714c4a286bee00e5be
	Issue introduced in 2.6.38 with commit cc53ce53c86924bfe98a12ea20b7465038a08792 and fixed in 6.18.50 with commit 467d56fd3ff57447a790c6dc3ede2d02a947d224
	Issue introduced in 2.6.38 with commit cc53ce53c86924bfe98a12ea20b7465038a08792 and fixed in 7.2.4 with commit 2bc4343308d85ee4e0dd3877b384306c96f114c2
	Issue introduced in 2.6.38 with commit cc53ce53c86924bfe98a12ea20b7465038a08792 and fixed in 7.3-rc1 with commit 6cba08dc1922140d260cfeb30bbda4ee1bf869d8

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-2026-89707
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/nfsd/vfs.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/194316df81263519156ebe714c4a286bee00e5be
	https://git.kernel.org/stable/c/467d56fd3ff57447a790c6dc3ede2d02a947d224
	https://git.kernel.org/stable/c/2bc4343308d85ee4e0dd3877b384306c96f114c2
	https://git.kernel.org/stable/c/6cba08dc1922140d260cfeb30bbda4ee1bf869d8