CVE-2026-89705: nfsd: restore rq_status_counter to even on all nfsd_dispatch() exit paths

[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: restore rq_status_counter to even on all nfsd_dispatch() exit paths

nfsd_dispatch() sets rq_status_counter to an odd value once a request has
been decoded, and back to an even value once it has been fully processed,
forming a seq-lock like protocol with the lockless reader in
nfsd_nl_rpc_status_get_dumpit().

Only the fully successful path restored the counter to even. The cache-hit
(RC_REPLY), drop (RC_DROPIT / RQ_DROPME) and encode-error paths all return
after the odd-valued store without ever bringing the counter back to even.
Once one of those paths is taken, rq_status_counter is left odd: the next
request's decode ORs in 1 (still odd) and only a subsequent successful
encode restores even. While stuck odd, the dumpit reader treats the rqstp
fields as stable and its retry check compares against the same unchanging
odd value, so it never detects concurrent mutation. This exposes actively
mutating fields (e.g. args->ops / args->opcnt during compound decode and
release) to the lockless reader, which can read past the end of the
8-element inline ops array.

Add a helper that advances the counter to the next even value and call it
on every return path that follows the odd-valued store. The decode-error
path is left untouched as it is reached before the counter is set odd.

The Linux kernel CVE team has assigned CVE-2026-89705 to this issue.


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

	Issue introduced in 6.7 with commit bd9d6a3efa9709e653aafbeb859289feccb8e70c and fixed in 7.2.4 with commit b08c30f08d57f30699d36809d92f0b683dca261f
	Issue introduced in 6.7 with commit bd9d6a3efa9709e653aafbeb859289feccb8e70c and fixed in 7.3-rc1 with commit f6045886fe3f14f269f683d64021b004a50d0efa

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-2026-89705
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/nfssvc.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/b08c30f08d57f30699d36809d92f0b683dca261f
	https://git.kernel.org/stable/c/f6045886fe3f14f269f683d64021b004a50d0efa