[PATCH 5/8] xprtrdma: Fix bcall rep leak and unbounded peek

From: Chris Mason <[email protected]>

rpcrdma_is_bcall() decodes a reply's first words to decide whether
the frame is a backchannel call. Two issues in that decode path
let a short or malformed reply leak the receive buffer and drain
the Receive queue.

First, the speculative peek

    p = xdr_inline_decode(xdr, 0);
    /* five p++ reads follow */

asks xdr_inline_decode() for zero bytes, which returns xdr->p
without consulting xdr->end. The five subsequent __be32 reads can
then walk up to 20 bytes past the wire payload into stale regbuf
contents and misclassify the reply as a backchannel call.

Second, after the post-peek

    p = xdr_inline_decode(xdr, 3 * sizeof(*p));
    if (unlikely(!p))
            return true;

the short-header arm returns true without calling
rpcrdma_bc_receive_call(). The contract with the caller is that a
true return transfers ownership of rep to the backchannel path:

    rpcrdma_reply_handler()
      if (rpcrdma_is_bcall(r_xprt, rep))
              return;        /* bare return, skips out_post */
      ...
    out_post:
      rpcrdma_post_recvs(r_xprt, credits + ...);

Because rpcrdma_bc_receive_call() never ran, no one took rep, but
rpcrdma_reply_handler still bare-returns past rpcrdma_rep_put()
and rpcrdma_post_recvs(). The rep, with its persistently
DMA-mapped receive buffer, is orphaned on rb_all_reps and freed
only at transport teardown. This completion reposts nothing, so
its slot is reclaimed only when a later forward-channel reply
reaches out_post and rpcrdma_post_recvs() allocates a fresh rep to
backfill; absent that traffic the Receive queue drains and the
peer's Sends draw RNR NAKs.

Fix by consulting xdr->end after the zero-length peek so the five
__be32 reads cannot run unless 20 bytes of wire payload remain. A
byte-precise comparison against xdr->end is required because a
non-4-aligned receive rounds the stream's word count up past the
true payload. Also return false from the short-header arm so the
reply falls through the normal out_norqst cleanup chain
(rpcrdma_rep_put() plus rpcrdma_post_recvs()).

Fixes: 41c8f70f5a3d ("xprtrdma: Harden backchannel call decoding")
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>
---
 net/sunrpc/xprtrdma/rpc_rdma.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
index f115baba6d56..63e64d53e289 100644
--- a/net/sunrpc/xprtrdma/rpc_rdma.c
+++ b/net/sunrpc/xprtrdma/rpc_rdma.c
@@ -1088,6 +1088,8 @@ rpcrdma_is_bcall(struct rpcrdma_xprt *r_xprt, struct rpcrdma_rep *rep)
 
 	/* Peek at stream contents without advancing. */
 	p = xdr_inline_decode(xdr, 0);
+	if ((char *)xdr->end - (char *)p < 5 * XDR_UNIT)
+		return false;
 
 	/* Chunk lists */
 	if (xdr_item_is_present(p++))
@@ -1112,7 +1114,7 @@ rpcrdma_is_bcall(struct rpcrdma_xprt *r_xprt, struct rpcrdma_rep *rep)
 	 */
 	p = xdr_inline_decode(xdr, 3 * sizeof(*p));
 	if (unlikely(!p))
-		return true;
+		return false;
 
 	rpcrdma_bc_receive_call(r_xprt, rep);
 	return true;

-- 
2.54.0