CVE-2026-89662: NFSD: Prevent lock owner use-after-free during client teardown

[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: Prevent lock owner use-after-free during client teardown

__destroy_client() releases a client's open owners, but a lock owner
whose only reference is a blocked lock (nbl) stays on
cl_ownerstr_hashtbl.  client_has_state() does not count a bare owner,
so DESTROY_CLIENTID can reach __destroy_client() with such owners
present.

__destroy_client() then walks the table, calling remove_blocked_locks()
on each owner without a reference.  Freeing a blocked lock drops the
owner reference held via flc_owner.  The per-net laundromat reaps
blocked locks from nn->blocked_locks_lru independently of client state.
The two paths share blocked_locks_lock only for the list splice, not
the owner's lifetime.  The laundromat therefore frees the owner as
__destroy_client() dereferences it, a NULL dereference in
remove_blocked_locks().

nfsd4_release_lockowner() holds a reference across the same call;
__destroy_client() does not.  Hold cl_lock across the walk, taking a
reference and unhashing each owner, then drop it before
remove_blocked_locks() and nfs4_put_stateowner(), which take
blocked_locks_lock and cl_lock.

The Linux kernel CVE team has assigned CVE-2026-89662 to this issue.


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

	Issue introduced in 4.16 with commit 68ef3bc3166468678d5e1fdd216628c35bd1186f and fixed in 6.12.109 with commit a6ead6fff3a7d03e49845f048f7000a2e0d34431
	Issue introduced in 4.16 with commit 68ef3bc3166468678d5e1fdd216628c35bd1186f and fixed in 6.18.50 with commit 4804c58f73a80d12df1de323bf97a164f6ee8743
	Issue introduced in 4.16 with commit 68ef3bc3166468678d5e1fdd216628c35bd1186f and fixed in 7.2.4 with commit 8cf4ff0a7c083dd5e0067f517d63979b74b66649
	Issue introduced in 4.16 with commit 68ef3bc3166468678d5e1fdd216628c35bd1186f and fixed in 7.3-rc1 with commit 5e2fa29d223a9a1e6a948e40b109d09081d1decd
	Issue introduced in 4.9.91 with commit e294c4c2d33b7307a89cdf31bec08a112d6a9297
	Issue introduced in 4.14.31 with commit 797bfd05d4040fbf766198b3cfcc1838002cc890
	Issue introduced in 4.15.14 with commit 0f44e9da465ea259b6c9f31a627f3c50a16e1679

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-2026-89662
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/nfs4state.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/a6ead6fff3a7d03e49845f048f7000a2e0d34431
	https://git.kernel.org/stable/c/4804c58f73a80d12df1de323bf97a164f6ee8743
	https://git.kernel.org/stable/c/8cf4ff0a7c083dd5e0067f517d63979b74b66649
	https://git.kernel.org/stable/c/5e2fa29d223a9a1e6a948e40b109d09081d1decd