[PATCH] rds: fix out-of-bounds conn_path access via handshake NPATHS

Connections on the IB transport (t_mp_capable unset) own exactly one
rds_conn_path slot, but the peer can still claim eight lanes through
the NPATHS extension header of a handshake packet, and every
subsequent fan-out path then treats all eight entries as real: the
mp-start helper iterates conn->c_path[0..7], dereferencing cp->cp_conn,
taking cp->cp_lock and queueing delayed work on cp->cp_wq at 504-byte
strides into whatever comes after the single allocated slot (KASAN:
"Read of size 8 in rds_conn_path_connect_if_down").

The negotiated lane count is now clamped to the same value the
transport was allocated with (trans->t_mp_capable ?
RDS_MPATH_WORKERS : 1), so a peer can never ask us to walk lanes that
do not exist.

This vulnerability was discovered by Tencent CodeBuddy Security.

Cc: stable@vger.kernel.org
Fixes: ba3d1f480c7a ("net/rds: size a connection's path set by the transport it ends up with")
Signed-off-by: Henry Martin <redacted>
---
 net/rds/recv.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/net/rds/recv.c
+++ b/net/rds/recv.c

@@ -224,7 +224,8 @@
 		/* Process extension header here */
 		switch (type) {
 		case RDS_EXTHDR_NPATHS:
-			new_npaths = min_t(int, RDS_MPATH_WORKERS,
+			new_npaths = min_t(int, conn->c_trans->t_mp_capable ?
+					   RDS_MPATH_WORKERS : 1,
 					   be16_to_cpu(buffer.rds_npaths));
 			break;
 		case RDS_EXTHDR_GEN_NUM:
--

2.43.7