CVE-2026-89679: nfsd: fix null dereference in nfsd4_setattr for deleg timestamp attrs

CVE-2026-89679: nfsd: fix null dereference in nfsd4_setattr for deleg timestamp attrs

[Greg Kroah-Hartman]

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

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

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

nfsd: fix null dereference in nfsd4_setattr for deleg timestamp attrs

When a SETATTR request includes FATTR4_WORD2_TIME_DELEG_ACCESS or
FATTR4_WORD2_TIME_DELEG_MODIFY in the attribute bitmap, nfsd4_setattr()
sets deleg_attrs=true and calls nfs4_preprocess_stateid_op() to validate
the stateid.

If the client supplies the NFSv4 "one stateid" (all-0xFF bytes),
check_special_stateids() returns nfs_ok without populating the output
nfs4_stid pointer, because the special-stateid path in
nfs4_preprocess_stateid_op() jumps to done: with s==NULL, and the
"if (s)" block that would set *cstid is skipped. The local variable `st`
remains NULL.

Back in nfsd4_setattr(), the if (deleg_attrs) block then unconditionally
dereferences st->sc_type (at offset 4 from NULL), causing a kernel oops.

This is remotely triggerable by any NFSv4 client: send COMPOUND [PUTROOTFH,
SETATTR(ONE_STATEID, {bmval2=FATTR4_WORD2_TIME_DELEG_ACCESS, ...})].
No authentication, delegation, or prior state is required.

Fix by adding a NULL check before the dereference. A special stateid is
not a delegation stateid, so the existing nfserr_bad_stateid return value
is already correct; we only need to guard the pointer dereference itself.

The Linux kernel CVE team has assigned CVE-2026-89679 to this issue.


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

	Issue introduced in 6.14 with commit 7e13f4f8d27dc02fb88666f603c53ca749d56f92 and fixed in 6.18.50 with commit 3c5119b799a7f57db332b2c31e806de16c869f0f
	Issue introduced in 6.14 with commit 7e13f4f8d27dc02fb88666f603c53ca749d56f92 and fixed in 7.2.4 with commit 3c896db123892256fd39af0ee8c2d02c63539983
	Issue introduced in 6.14 with commit 7e13f4f8d27dc02fb88666f603c53ca749d56f92 and fixed in 7.3-rc1 with commit fe456c8c0931bb3e8a03d429920e87fd85747fba

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-2026-89679
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/3c5119b799a7f57db332b2c31e806de16c869f0f
	https://git.kernel.org/stable/c/3c896db123892256fd39af0ee8c2d02c63539983
	https://git.kernel.org/stable/c/fe456c8c0931bb3e8a03d429920e87fd85747fba