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svcrdma: Reject Write/Reply chunks with segcount 0

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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: 78147ca8b4 ("svcrdma: Add a "parsed chunk list" data structure")
Cc: stable@vger.kernel.org
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260526-rpc-kernel-bugs-v1-5-e251306ccca9@oracle.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
This commit is contained in:
Chris Mason 2026-05-26 09:35:59 -04:00 committed by Chuck Lever
parent b7713a784c
commit 9808eb7656
2 changed files with 7 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
net/sunrpc/xprtrdma/svc_rdma_pcl.c
View File

@ -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;
}

9
net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
View File

@ -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,