CVE-2026-89665: nfsd: reject out-of-range useconds in NFSv2 SETATTR/CREATE

CVE-2026-89665: nfsd: reject out-of-range useconds in NFSv2 SETATTR/CREATE

[Greg Kroah-Hartman]

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

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

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

nfsd: reject out-of-range useconds in NFSv2 SETATTR/CREATE

The NFSv2 sattr decoder converts the wire useconds to nanoseconds in
svcxdr_decode_sattr():

	iap->ia_atime.tv_nsec = tmp2 * NSEC_PER_USEC;

tmp2 is a u32 and NSEC_PER_USEC is 1000, so the product is computed in
unsigned long. On ILP32 that is 32 bits, and an out-of-range useconds
value such as 4294968 wraps to tv_nsec == 704. The corruption therefore
happens during decode, before any proc function can inspect the value,
and a later range check on tv_nsec would see an in-range result and
accept it. Rejecting in the decoder yields an RPC GARBAGE_ARGS reply.
NFSv2 defines no NFSERR_INVAL, so there is no NFS-level status to return
for a malformed time argument, and the check cannot move to the proc
function the way the v3/v4 nsec range checks do.

Guard the raw useconds before the multiplication and reject values
greater than 1000000. useconds == 1000000 is kept: it is the Sun
convention for "set to the current server time", and the in-tree Linux
NFSv2 client emits it in both the atime and the mtime field for a plain
touch / utimes(file, NULL) (see encode_sattr() and
xdr_encode_current_server_time() in fs/nfs/nfs2xdr.c). Rejecting 1000000
would turn that common operation into a hard decode failure for both
SETATTR and CREATE. 1000000 * NSEC_PER_USEC is 10^9, which does not wrap
on ILP32, so the Sun convention value passes through safely. Only
genuinely out-of-range values (> 1000000) are rejected. The atime and
mtime guards are therefore symmetric.

The decoder only applied the Sun convention in the mtime block, which
clears ATTR_ATIME_SET|ATTR_MTIME_SET when mtime useconds == 1000000. If a
client puts 1000000 in the atime field but not in the mtime field, the
atime block stored an out-of-range tv_nsec (10^9) and left ATTR_ATIME_SET
set, so the bogus value reached the filesystem. Apply the convention in
the atime block as well, clearing ATTR_ATIME_SET so the server uses its
current time and ignores the value. Only ATTR_ATIME_SET is cleared there.
The mtime block keeps its existing behavior, where 1000000 means "set
both atime and mtime to now".

[ cel: various tweaks, addenda, and clean-ups ]

The Linux kernel CVE team has assigned CVE-2026-89665 to this issue.


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

	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.12.109 with commit a937dd1aa2d92291fe0a1860d17e90d8f206cd22
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.18.50 with commit ad02d095439f89cbd6629420c7e3ba476457b83b
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.2.4 with commit 1195483965a5f63d2dde18054ec7e50a23b9071c
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.3-rc1 with commit 26709c8ffe73772eb69e68d553ac71d91228dccc

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-2026-89665
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/nfsxdr.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/a937dd1aa2d92291fe0a1860d17e90d8f206cd22
	https://git.kernel.org/stable/c/ad02d095439f89cbd6629420c7e3ba476457b83b
	https://git.kernel.org/stable/c/1195483965a5f63d2dde18054ec7e50a23b9071c
	https://git.kernel.org/stable/c/26709c8ffe73772eb69e68d553ac71d91228dccc