CVE-2026-89696: nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref

[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: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref

When CONFIG_NFSD_V4_2_INTER_SSC is enabled, nfsd4_putfh() can return
success with fh_dentry and fh_export both NULL if fh_verify() returns
nfserr_stale and putfh->no_verify is true. The NFSD4_FH_FOREIGN flag
is set, but the compound dispatch loop only uses this flag to bypass
the nfserr_nofilehandle check -- it does not prevent subsequent ops
from running with a NULL fh_dentry.

A remote client can exploit this by crafting a COMPOUND that includes
an inter-SSC COPY (which causes check_if_stalefh_allowed() to set
no_verify=true on the saved PUTFH) with an additional op inserted
between the source PUTFH and SAVEFH. For example, SETATTR calls
fh_want_write() which dereferences fh_export->ex_path.mnt without
calling fh_verify() first, causing a NULL pointer dereference in the
nfsd kthread.

Fix this by gating the dispatch loop: when NFSD4_FH_FOREIGN is set
and fh_dentry is NULL, only OP_SAVEFH (needed for the inter-SSC flow)
and ops with ALLOWED_WITHOUT_FH (which don't need a resolved
filehandle) may proceed. All other ops receive nfserr_stale, per
RFC 7862 Section 15.2.3 which specifies that foreign filehandle
validation is deferred to the consuming operation and NFS4ERR_STALE
returned at that point.

The Linux kernel CVE team has assigned CVE-2026-89696 to this issue.


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

	Issue introduced in 5.6 with commit b9e8638e3d9ed8334f1f7071e081860aac37e83e and fixed in 6.12.109 with commit 35f248bd40b47b229d4999581df45b97daadd977
	Issue introduced in 5.6 with commit b9e8638e3d9ed8334f1f7071e081860aac37e83e and fixed in 6.18.50 with commit 311f7d926630940650447cbd1c932b076b40a6c4
	Issue introduced in 5.6 with commit b9e8638e3d9ed8334f1f7071e081860aac37e83e and fixed in 7.2.4 with commit bf4d338dc8625d70c7f2cb0657d66851a7ac9154
	Issue introduced in 5.6 with commit b9e8638e3d9ed8334f1f7071e081860aac37e83e and fixed in 7.3-rc1 with commit c59738a00aa51b16adc1b5ceb7c80877168efb4d

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-2026-89696
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/nfs4proc.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/35f248bd40b47b229d4999581df45b97daadd977
	https://git.kernel.org/stable/c/311f7d926630940650447cbd1c932b076b40a6c4
	https://git.kernel.org/stable/c/bf4d338dc8625d70c7f2cb0657d66851a7ac9154
	https://git.kernel.org/stable/c/c59738a00aa51b16adc1b5ceb7c80877168efb4d