FAILED: patch "[PATCH] svcrdma: Reject Write/Reply chunks with segcount 0" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 9808eb7656666acc7291bae9ab6b987bd16e47e0
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090342-canary-synopses-a220@gregkh' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 9808eb7656666acc7291bae9ab6b987bd16e47e0 Mon Sep 17 00:00:00 2001
From: Chris Mason <[email protected]>
Date: Tue, 26 May 2026 09:35:59 -0400
Subject: [PATCH] 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.

index 1f8f7dad8b6f..18d1045799ce 100644
@@ -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;
 }

index 15c1d8ae5301..f6a7533a7555 100644
@@ -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,

From: Chuck Lever <[email protected]>

[ Upstream commit eff042ddf4b9587fa7394d502b93e06ec6015177 ]

This page count is to be used to allocate various arrays of pages and bio_vecs, replacing the fixed RPCSVC_MAXPAGES value.

The documenting comment is somewhat stale -- of course NFSv4 COMPOUND procedures may have multiple payloads.

Stable-dep-of: 9808eb765666 ("svcrdma: Reject Write/Reply chunks with segcount 0")

---
 include/linux/sunrpc/svc.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

index 617ebfff2f304..2285fd735eacc 100644
@@ -163,6 +163,23 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp);
 #define RPCSVC_MAXPAGES		((RPCSVC_MAXPAYLOAD+PAGE_SIZE-1)/PAGE_SIZE \
 				+ 2 + 1)
 
+/**
+ * svc_serv_maxpages - maximum count of pages needed for one RPC message
+ * @serv: RPC service context
+ *
+ * Returns a count of pages or vectors that can hold the maximum
+ * size RPC message for @serv.
+ *
+ * Each request/reply pair can have at most one "payload", plus two
+ * pages, one for the request, and one for the reply.
+ * nfsd_splice_actor() might need an extra page when a READ payload
+ * is not page-aligned.
+ */
+static inline unsigned long svc_serv_maxpages(const struct svc_serv *serv)
+{
+	return DIV_ROUND_UP(serv->sv_max_mesg, PAGE_SIZE) + 2 + 1;
+}
+
 /*
  * The context of a single thread, including the request currently being
  * processed.

2.53.0

From: Chuck Lever <[email protected]>

[ Upstream commit 81381d1a90dea186ce4585d9bd3cdc40b50e9c48 ]

Allow allocation of more entries in the rc_pages[] array when the maximum size of an RPC message is increased.

Stable-dep-of: 9808eb765666 ("svcrdma: Reject Write/Reply chunks with segcount 0")

---
 include/linux/sunrpc/svc_rdma.h         | 3 ++-
 net/sunrpc/xprtrdma/svc_rdma_recvfrom.c | 8 ++++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

index 619fc0bd837a8..1016f2feddc4b 100644
@@ -202,7 +202,8 @@ struct svc_rdma_recv_ctxt {
 	struct svc_rdma_pcl	rc_reply_pcl;
 
 	unsigned int		rc_page_count;
-	struct page		*rc_pages[RPCSVC_MAXPAGES];
+	unsigned long		rc_maxpages;
+	struct page		*rc_pages[] __counted_by(rc_maxpages);
 };
 
 /*

index 292022f0976e1..e7e4a39ca6c65 100644
@@ -120,12 +120,16 @@ svc_rdma_recv_ctxt_alloc(struct svcxprt_rdma *rdma)
 {
 	int node = ibdev_to_node(rdma->sc_cm_id->device);
 	struct svc_rdma_recv_ctxt *ctxt;
+	unsigned long pages;
 	dma_addr_t addr;
 	void *buffer;
 
-	ctxt = kzalloc_node(sizeof(*ctxt), GFP_KERNEL, node);
+	pages = svc_serv_maxpages(rdma->sc_xprt.xpt_server);
+	ctxt = kzalloc_node(struct_size(ctxt, rc_pages, pages),
+			    GFP_KERNEL, node);
 	if (!ctxt)
 		goto fail0;
+	ctxt->rc_maxpages = pages;
 	buffer = kmalloc_node(rdma->sc_max_req_size, GFP_KERNEL, node);
 	if (!buffer)
 		goto fail1;

@@ -497,7 +501,7 @@ static bool xdr_check_write_chunk(struct svc_rdma_recv_ctxt *rctxt)
 	 * a computation, perform a simple range check. This is an
 	 * arbitrary but sensible limit (ie, not architectural).
 	 */
-	if (unlikely(segcount > RPCSVC_MAXPAGES))
+	if (unlikely(segcount > rctxt->rc_maxpages))
 		return false;
 
 	p = xdr_inline_decode(&rctxt->rc_stream,

2.53.0

From: Chris Mason <[email protected]>

[ Upstream commit 9808eb7656666acc7291bae9ab6b987bd16e47e0 ]

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.

---
 net/sunrpc/xprtrdma/svc_rdma_pcl.c      | 2 +-
 net/sunrpc/xprtrdma/svc_rdma_recvfrom.c | 9 ++++++---
 2 files changed, 7 insertions(+), 4 deletions(-)

index b63cfeaa29234..a1c61311714da 100644
@@ -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;
 }

index e7e4a39ca6c65..68a0aa627455d 100644
@@ -498,10 +498,13 @@ 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,

2.53.0