CVE-2026-93229: nfsd: add missing read barrier to rpc_status_get dumpit seqcount retry

[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: add missing read barrier to rpc_status_get dumpit seqcount retry

The hand-rolled seqcount-like protocol in nfsd_nl_rpc_status_get_dumpit()
is missing a read memory barrier (smp_rmb) before its second counter
check.  The standard kernel read_seqcount_retry() includes smp_rmb()
to ensure that all data reads complete before the counter is re-checked.

Without this barrier, on weakly-ordered architectures (ARM, POWER),
the CPU may reorder field reads past the second counter check, making
the retry logic ineffective: it could observe a consistent counter pair
while reading fields that have been concurrently modified by the writer.

Add smp_rmb() before the second counter check to order the field reads
ahead of it, matching the barrier semantics of the standard seqcount
read-side.  The begin-side smp_load_acquire() already pairs with the
smp_store_release() in nfsd_dispatch(); with the smp_rmb() now ordering
the field reads, the retry check no longer needs acquire semantics and
reads the counter with a plain READ_ONCE(), as read_seqcount_retry()
does.

[ cel: Use READ_ONCE instead of smp_load_acquire() ]

The Linux kernel CVE team has assigned CVE-2026-93229 to this issue.


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

	Issue introduced in 6.7 with commit bd9d6a3efa9709e653aafbeb859289feccb8e70c and fixed in 6.12.109 with commit f501f2f4ec1d2dfe39e21c98630314074a9b30b0
	Issue introduced in 6.7 with commit bd9d6a3efa9709e653aafbeb859289feccb8e70c and fixed in 6.18.50 with commit 1aea0482b98ecd7d0249204665f2ad4ad517f66b
	Issue introduced in 6.7 with commit bd9d6a3efa9709e653aafbeb859289feccb8e70c and fixed in 7.2.4 with commit 9b5f6475006cd8e3b5b99b8eb3cd74dbb1ce9df8
	Issue introduced in 6.7 with commit bd9d6a3efa9709e653aafbeb859289feccb8e70c and fixed in 7.3-rc1 with commit a71f161a857117e8e0264deb7d14fff5c98adcf5

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-2026-93229
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/nfsctl.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/f501f2f4ec1d2dfe39e21c98630314074a9b30b0
	https://git.kernel.org/stable/c/1aea0482b98ecd7d0249204665f2ad4ad517f66b
	https://git.kernel.org/stable/c/9b5f6475006cd8e3b5b99b8eb3cd74dbb1ce9df8
	https://git.kernel.org/stable/c/a71f161a857117e8e0264deb7d14fff5c98adcf5