[PATCH] can: j1939: fix memory leak of skbs

[Fedor Pchelkin <pchelkin@ispras.ru>]

Syzkaller reported memory leak of skbs introduced with the commit
2030043e616c ("can: j1939: fix Use-after-Free, hold skb ref while in use").

Link to Syzkaller info and repro: https://forge.ispras.ru/issues/11743

The suggested solution was tested on the new memory-leak Syzkaller repro
and on the old use-after-free repro (that use-after-free bug was solved
with aforementioned commit). Although there can probably be another
situations when the numbers of skb_get() and skb_unref() calls don't match
and I don't see it in right way.

Moreover, skb_unref() call can be harmlessly removed from line 338 in
j1939_session_skb_drop_old() (/net/can/j1939/transport.c). But then I
assume this removal ruins the whole reference counts logic...

Overall, there is definitely something not clear in skb reference counts
management with skb_get() and skb_unref(). The solution we suggested fixes
the leaks and use-after-free's induced by Syzkaller but perhaps the origin
of the problem can be somewhere else.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
---
 net/can/j1939/transport.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index 307ee1174a6e..9600b339cbf8 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -356,7 +356,6 @@ void j1939_session_skb_queue(struct j1939_session *session,
 
 	skcb->flags |= J1939_ECU_LOCAL_SRC;
 
-	skb_get(skb);
 	skb_queue_tail(&session->skb_queue, skb);
 }
 
-- 
2.25.1