[PATCH net v8 3/4] can: j1939: drop the message when the reassembled skb is gone

[Chaosheng Qu <quchaosheng000406@163.com>]

j1939_session_completed() hands the result of j1939_session_skb_get() to
j1939_sk_recv() without checking it, and j1939_sk_recv() dereferences
oskb->sk.  A NULL here is the last step of the EDPO out-of-range sequence
fixed by the previous patch; the bounds check makes the lookup succeed,
but the dereference should not depend on that.

Drop the message instead, which is what the other caller of
j1939_session_skb_get(), j1939_simple_txnext(), already does.

Signed-off-by: Chaosheng Qu <quchaosheng000406@163.com>
---
 net/can/j1939/transport.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index ab42a383ff87..124054668650 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -1281,9 +1281,17 @@ static void j1939_session_completed(struct j1939_session *session)
 
 	if (!session->transmission) {
 		se_skb = j1939_session_skb_get(session);
-		/* distribute among j1939 receivers */
-		j1939_sk_recv(session->priv, se_skb);
-		consume_skb(se_skb);
+
+		/* Defense in depth: pkt.dpo is range checked when an EDPO is
+		 * received, so the lookup should always find the skb.  Drop
+		 * the message rather than hand a NULL skb to j1939_sk_recv()
+		 * if the offset arithmetic ever regresses.
+		 */
+		if (se_skb) {
+			/* distribute among j1939 receivers */
+			j1939_sk_recv(session->priv, se_skb);
+			consume_skb(se_skb);
+		}
 	}
 
 	j1939_session_deactivate_activate_next(session);
-- 
2.43.0