[PATCH] rds: fix out-of-bounds conn_path walk in peer gen update

[Henry Martin <bsdhenrymartin@gmail.com>]

A connection on the IB transport (t_mp_capable unset) is allocated
with exactly one rds_conn_path slot, yet the peer's genetic number
update routine is hardcoded to walk RDS_MPATH_WORKERS entries whenever
the peer announces a non-zero gen number that differs from the one we
cached.  Everything past the first entry lands on whatever happens to
follow the single-slot array in the same slab slot group, and the loop
then reads and modifies struct fields there (lock acquired,
cp_next_tx_seq cleared, retransmit list inspected).  A malicious peer
can therefore force consecutive out-of-bounds writes into neighbouring
kmalloc-512 objects (KASAN: "Write of size 4 in
_raw_spin_lock_irqsave").

The allocation and the walk now agree on the same expression
(trans->t_mp_capable ? RDS_MPATH_WORKERS : 1), so the loop can never
outlive the array regardless of what the peer reports.

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 <bsdhenrymartin@gmail.com>
---
 net/rds/recv.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/net/rds/recv.c
+++ b/net/rds/recv.c
@@ -144,7 +144,8 @@
 	if (peer_gen_num != 0) {
 		if (conn->c_peer_gen_num != 0 &&
 		    peer_gen_num != conn->c_peer_gen_num) {
-			for (i = 0; i < RDS_MPATH_WORKERS; i++) {
+			for (i = 0; i < (conn->c_trans->t_mp_capable ?
+			     RDS_MPATH_WORKERS : 1); i++) {
 				struct rds_conn_path *cp;

 				cp = &conn->c_path[i];
--
2.43.7