CVE-2026-93228: svcrdma: Reject Write/Reply chunks with segcount 0

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

svcrdma: Reject Write/Reply chunks with segcount 0

A peer can send a Write or Reply chunk whose segcount field is zero.
xdr_check_write_chunk() only rejects segcount > rc_maxpages, so zero
passes the range check, and xdr_inline_decode(stream, 0) returns the
current (non-NULL) cursor without advancing. The function returns
true and pcl_alloc_write() then links a struct svc_rdma_chunk with
ch_segcount == 0 onto rc_write_pcl or rc_reply_pcl.

An earlier patch in this series made pcl_for_each_segment() safe for
ch_segcount == 0, so this no longer drives the memory walk it used
to. Rejecting the malformed frame at the decode boundary is still
worthwhile as defense in depth: it keeps degenerate zero-segment
chunks off the parsed chunk lists entirely, so any future consumer
that walks ch_segments directly cannot observe one, and it makes the
zero-floor easy to backport to trees where the macro change is more
intrusive. RFC 8166 has no meaning for a Write/Reply chunk that
describes no remote buffer, so no legitimate client is affected.

xdr_check_reply_chunk() funnels Reply chunks through
xdr_check_write_chunk() and inherits the same rejection.

pcl_alloc_write() also links each chunk onto the parsed chunk list
before filling its segment array. If a future change weakens the
segcount-0 rejection, an incomplete chunk is visible to consumers
during the fill loop. Reorder so that list_add_tail() follows the
segment fill loop, ensuring only fully-populated chunks appear on
the list.

The Linux kernel CVE team has assigned CVE-2026-93228 to this issue.


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

	Issue introduced in 5.11 with commit 78147ca8b4a9b6cf0e597ddd6bf17959e08376c2 and fixed in 6.12.111 with commit fe533ae1bed2ef2b3d54ee7ab410da1874ed3d4f
	Issue introduced in 5.11 with commit 78147ca8b4a9b6cf0e597ddd6bf17959e08376c2 and fixed in 6.18.50 with commit a798714b58041e88716db3bf8fb03ae13eecd54a
	Issue introduced in 5.11 with commit 78147ca8b4a9b6cf0e597ddd6bf17959e08376c2 and fixed in 7.2.4 with commit 45dbdb2637b7fc5f1355b588780d2a7fb0516805
	Issue introduced in 5.11 with commit 78147ca8b4a9b6cf0e597ddd6bf17959e08376c2 and fixed in 7.3-rc1 with commit 9808eb7656666acc7291bae9ab6b987bd16e47e0

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-2026-93228
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:
	net/sunrpc/xprtrdma/svc_rdma_pcl.c
	net/sunrpc/xprtrdma/svc_rdma_recvfrom.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/fe533ae1bed2ef2b3d54ee7ab410da1874ed3d4f
	https://git.kernel.org/stable/c/a798714b58041e88716db3bf8fb03ae13eecd54a
	https://git.kernel.org/stable/c/45dbdb2637b7fc5f1355b588780d2a7fb0516805
	https://git.kernel.org/stable/c/9808eb7656666acc7291bae9ab6b987bd16e47e0