CVE-2026-89688: nfsd: drop the stateid, not the stateowner, on seqid_op replay retry

CVE-2026-89688: nfsd: drop the stateid, not the stateowner, on seqid_op replay retry

[Greg Kroah-Hartman]

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

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

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

nfsd: drop the stateid, not the stateowner, on seqid_op replay retry

In nfs4_preprocess_seqid_op() the stateid is obtained from
nfsd4_lookup_stateid(), which holds a reference on the nfs4_stid
(sc_count) but takes no reference on the stateowner. openlockstateid()
merely casts that stid and likewise takes no reference.

When nfsd4_cstate_assign_replay() returns -EAGAIN (the replay owner is
being torn down, RP_UNHASHED) it has not taken a stateowner reference on
that path. The error handling nevertheless called
nfs4_put_stateowner(stp->st_stateowner), dropping an so_count reference
the function never acquired -- risking a stateowner refcount underflow and
use-after-free -- while leaking the sc_count reference held on the stid.
The leaked stid reference can also stall a concurrent
nfsd4_close_open_stateid() waiting for sc_count to drop.

Drop the reference actually held -- the stid -- before retrying. The
stateowner stays alive through the reference held by the stid. This mirrors
the open path in nfsd4_process_open1(), where the put balances a reference
that path explicitly holds on the stateowner.

The Linux kernel CVE team has assigned CVE-2026-89688 to this issue.


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

	Issue introduced in 6.10 with commit eec7620800081e27dbf8019ac2e66259f0d5bf6f and fixed in 6.12.109 with commit 69ed78b6b947c9257213164678b4edf17533093b
	Issue introduced in 6.10 with commit eec7620800081e27dbf8019ac2e66259f0d5bf6f and fixed in 6.18.50 with commit 00843074d9b84824552c9679d423d29500ca5de0
	Issue introduced in 6.10 with commit eec7620800081e27dbf8019ac2e66259f0d5bf6f and fixed in 7.2.4 with commit f7cb90ddc021747fc9abfd4cf5252d425fd34eec
	Issue introduced in 6.10 with commit eec7620800081e27dbf8019ac2e66259f0d5bf6f and fixed in 7.3-rc1 with commit 5e4627d3513e60accfce9d5f4c7fa95251ef93d6

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-2026-89688
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/69ed78b6b947c9257213164678b4edf17533093b
	https://git.kernel.org/stable/c/00843074d9b84824552c9679d423d29500ca5de0
	https://git.kernel.org/stable/c/f7cb90ddc021747fc9abfd4cf5252d425fd34eec
	https://git.kernel.org/stable/c/5e4627d3513e60accfce9d5f4c7fa95251ef93d6