[PATCH 5/6] svcrdma: reject Write/Reply chunks with segcount 0

[Chuck Lever <cel@kernel.org>]

From: Chris Mason <clm@meta.com>

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.

Fixes: 78147ca8b4a9 ("svcrdma: Add a "parsed chunk list" data structure")
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/xprtrdma/svc_rdma_pcl.c      | 2 +-
 net/sunrpc/xprtrdma/svc_rdma_recvfrom.c | 9 ++++++---
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/sunrpc/xprtrdma/svc_rdma_pcl.c b/net/sunrpc/xprtrdma/svc_rdma_pcl.c
index 1f8f7dad8b6f..18d1045799ce 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_pcl.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_pcl.c
@@ -213,7 +213,6 @@ bool pcl_alloc_write(struct svc_rdma_recv_ctxt *rctxt,
 		chunk = pcl_alloc_chunk(segcount, 0);
 		if (!chunk)
 			return false;
-		list_add_tail(&chunk->ch_list, &pcl->cl_chunks);
 
 		for (j = 0; j < segcount; j++) {
 			segment = &chunk->ch_segments[j];
@@ -225,6 +224,7 @@ bool pcl_alloc_write(struct svc_rdma_recv_ctxt *rctxt,
 			chunk->ch_length += segment->rs_length;
 			chunk->ch_segcount++;
 		}
+		list_add_tail(&chunk->ch_list, &pcl->cl_chunks);
 	}
 	return true;
 }
diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
index 15c1d8ae5301..f6a7533a7555 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
@@ -510,10 +510,13 @@ static bool xdr_check_write_chunk(struct svc_rdma_recv_ctxt *rctxt)
 		return false;
 
 	/* Before trusting the segcount value enough to use it in
-	 * a computation, perform a simple range check. This is an
-	 * arbitrary but sensible limit (ie, not architectural).
+	 * a computation, perform a simple range check. A zero
+	 * segcount describes no remote buffer and is rejected so
+	 * downstream consumers never see a degenerate ch_segcount==0
+	 * chunk. The upper bound is an arbitrary but sensible limit
+	 * (ie, not architectural).
 	 */
-	if (unlikely(segcount > rctxt->rc_maxpages))
+	if (segcount == 0 || unlikely(segcount > rctxt->rc_maxpages))
 		return false;
 
 	p = xdr_inline_decode(&rctxt->rc_stream,

-- 
2.54.0