CVE-2026-89706: nfsd: Reset write verifier when async COPY writeback fails

CVE-2026-89706: nfsd: Reset write verifier when async COPY writeback fails

[Greg Kroah-Hartman]

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

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

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

nfsd: Reset write verifier when async COPY writeback fails

Async COPY captures nn->writeverf at request time and reports it to
the client via CB_OFFLOAD after the worker kthread completes. When
the post-copy vfs_fsync_range() or filemap_check_wb_err() in
_nfsd_copy_file_range() reports an error, the worker correctly
leaves NFSD4_COPY_F_COMMITTED clear so that CB_OFFLOAD encodes
wr_stable_how as NFS_UNSTABLE, but the server's write verifier is
not rotated.

A client that receives NFS_UNSTABLE in CB_OFFLOAD follows up with
COMMIT to make the copied data durable. With the verifier
unchanged, COMMIT returns the same value the client just received
via CB_OFFLOAD, and the client concludes the copy is durable --
silently dropping the data whose writeback in fact failed. This
violates the UNSTABLE+COMMIT durability contract (RFC 7862 section
15.1, RFC 8881 section 18.32) and matches the bug just fixed in
nfsd_vfs_write() and nfsd_commit().

Rotate nn->writeverf at the writeback-failure site. The async COPY
worker has no svc_rqst, so commit_reset_write_verifier() is not
available here; calling nfsd_reset_write_verifier() directly
mirrors the trace-less reset already used by
nfsd_file_check_write_error() for the same purpose. Filter out
-EAGAIN and -ESTALE, matching commit_reset_write_verifier(), since
neither indicates a durable-storage failure.

The Linux kernel CVE team has assigned CVE-2026-89706 to this issue.


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

	Issue introduced in 5.14 with commit eac0b17a77fbd763d305a5eaa4fd1119e5a0fe0d and fixed in 6.12.109 with commit 31d4d0a62ec4bafbcdbad142f11ab1f90794b1d0
	Issue introduced in 5.14 with commit eac0b17a77fbd763d305a5eaa4fd1119e5a0fe0d and fixed in 6.18.50 with commit 591134e059e3456180244a826d6917cdc183140b
	Issue introduced in 5.14 with commit eac0b17a77fbd763d305a5eaa4fd1119e5a0fe0d and fixed in 7.2.4 with commit bf0cd31a9abcb728c17d79b4c2cc533612a8b6af
	Issue introduced in 5.14 with commit eac0b17a77fbd763d305a5eaa4fd1119e5a0fe0d and fixed in 7.3-rc1 with commit f5cb2276954cb80987a93ef9f9dfbfdbfc0f10b9
	Issue introduced in 5.10.220 with commit 817c6eb975798647c4bbbfc9eb288aa413ec7f65

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-2026-89706
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/31d4d0a62ec4bafbcdbad142f11ab1f90794b1d0
	https://git.kernel.org/stable/c/591134e059e3456180244a826d6917cdc183140b
	https://git.kernel.org/stable/c/bf0cd31a9abcb728c17d79b4c2cc533612a8b6af
	https://git.kernel.org/stable/c/f5cb2276954cb80987a93ef9f9dfbfdbfc0f10b9