CVE-2026-89661: NFSD: Prevent post-shutdown use-after-free in unlock_filesystem

CVE-2026-89661: NFSD: Prevent post-shutdown use-after-free in unlock_filesystem

[Greg Kroah-Hartman]

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

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

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

NFSD: Prevent post-shutdown use-after-free in unlock_filesystem

Writing a filesystem path to /proc/fs/nfsd/unlock_filesystem runs
nfsd4_cancel_copy_by_sb() before nfsd_mutex is held and before the
handler confirms that nn->nfsd_serv is set. Once nfsd has shut down,
nfs4_state_destroy_net() has freed nn->conf_id_hashtbl but left the
pointer intact, so the cancel helper iterates freed slab memory as an
array of struct list_head and then dereferences a bogus nfs4_client
when it takes clp->async_lock. A local administrator holding
CAP_SYS_ADMIN can reach this use-after-free by stopping the server and
then writing to unlock_filesystem; KASAN reports a slab-use-after-free
read in nfsd4_cancel_copy_by_sb().

nfsd4_revoke_states() walks the same state tables and for that reason
already runs only under nfsd_mutex with nn->nfsd_serv confirmed
present. Move the async COPY cancel into that protected section so
every NFSv4 state-table walker on this path observes a running server.
Async copies exist only while the server runs, so gating the cancel on
nn->nfsd_serv loses nothing.

The Linux kernel CVE team has assigned CVE-2026-89661 to this issue.


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

	Issue introduced in 7.0 with commit 3daab3112f039cf849f96764019b096bb0a39d04 and fixed in 7.2.4 with commit 0c1a755b7212e0835398d4df5e782ea85ccd7476
	Issue introduced in 7.0 with commit 3daab3112f039cf849f96764019b096bb0a39d04 and fixed in 7.3-rc1 with commit 292d915d3ba6fd15eeb88351fa10581683073109

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-2026-89661
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/nfsctl.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/0c1a755b7212e0835398d4df5e782ea85ccd7476
	https://git.kernel.org/stable/c/292d915d3ba6fd15eeb88351fa10581683073109