RDMA/rtrs-srv: Bound RDMA-Write length to chunk size in rdma_write_sg

commit 963af8d97a upstream.

When the server answers an RTRS READ, rdma_write_sg() builds the source
scatter/gather entry for the IB_WR_RDMA_WRITE that returns data to the
peer. Its length is taken directly from the wire descriptor:

  plist->length = le32_to_cpu(id->rd_msg->desc[0].len);

rd_msg points into the chunk buffer that the remote peer filled via
RDMA-WRITE-WITH-IMM (rtrs_srv_rdma_done() -> process_io_req() ->
process_read()), so desc[0].len is attacker-controlled and, before this
change, was only rejected when zero. The source address is the fixed
chunk start (dma_addr[msg_id]) and the source lkey is the PD-wide
local_dma_lkey, which is not tied to the chunk's MR mapping, so the verbs
layer does not constrain the transfer length to max_chunk_size. msg_id
and off are bounded against queue_depth and max_chunk_size in
rtrs_srv_rdma_done(), but desc[0].len is a separate field that was not
checked against the chunk size.

A peer that advertises desc[0].len larger than max_chunk_size can make
the posted RDMA write read past the chunk's mapped region. The resulting
behaviour depends on the IOMMU configuration: with no IOMMU or in
passthrough mode the read may extend into memory adjacent to the chunk
and be returned to the peer, which can disclose host memory; with a
translating IOMMU the out-of-range access is expected to fault and abort
the connection. In either case the transfer exceeds what the protocol
permits and is driven by a remote peer.

Reject a descriptor length above max_chunk_size, mirroring the existing
off >= max_chunk_size bound in rtrs_srv_rdma_done(). Legitimate clients
do not exceed it: the client sets desc[0].len to its MR length, which is
capped at the negotiated max_io_size (max_chunk_size - MAX_HDR_SIZE).

Fixes: 9cb8374804 ("RDMA/rtrs: server: main functionality")
Link: https://patch.msgid.link/r/20260612-master-v1-1-70cde5c6fdc9@gmail.com
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Zhenhao Wan <whi4ed0g@gmail.com>
Reviewed-by: Md Haris Iqbal <haris.iqbal@ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/infiniband/ulp/rtrs/rtrs-srv.c

@@ -224,8 +224,9 @@ static int rdma_write_sg(struct rtrs_srv_op *id)
 	/* WR will fail with length error
 	 * if this is 0
 	 */
-	if (plist->length == 0) {
-		rtrs_err(s, "Invalid RDMA-Write sg list length 0\n");
+	if (plist->length == 0 || plist->length > max_chunk_size) {
+		rtrs_err(s, "Invalid RDMA-Write sg list length %u\n",
+			 plist->length);
 		return -EINVAL;
 	}