[PATCH net] can: j1939: fix errant WARN_ON_ONCE in j1939_session_deactivate

Ziyang Xuan
2021-09-06T09:42:58Z

The conclusion "j1939_session_deactivate() should be called with a
session ref-count of at least 2" is incorrect. In some concurrent
scenarios, j1939_session_deactivate can be called with the session
ref-count less than 2. But there is not any problem because it
will check the session active state before session putting in
j1939_session_deactivate_locked().

Here is the concurrent scenario of the problem reported by syzbot
and my reproduction log.

cpu0 cpu1
j1939_xtp_rx_eoma
j1939_xtp_rx_abort_one
j1939_session_get_by_addr [kref == 2]
j1939_session_get_by_addr [kref == 3]
j1939_session_deactivate [kref == 2]
j1939_session_put [kref == 1]
j1939_session_completed
j1939_session_deactivate
WARN_ON_ONCE(kref < 2)

=====================================================
WARNING: CPU: 1 PID: 21 at net/can/j1939/transport.c:1088 j1939_session_deactivate+0x5f/0x70
CPU: 1 PID: 21 Comm: ksoftirqd/1 Not tainted 5.14.0-rc7+ #32
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1 04/01/2014
RIP: 0010:j1939_session_deactivate+0x5f/0x70
Call Trace:
j1939_session_deactivate_activate_next+0x11/0x28
j1939_xtp_rx_eoma+0x12a/0x180
j1939_tp_recv+0x4a2/0x510
j1939_can_recv+0x226/0x380
can_rcv_filter+0xf8/0x220
can_receive+0x102/0x220
? process_backlog+0xf0/0x2c0
can_rcv+0x53/0xf0
__netif_receive_skb_one_core+0x67/0x90
? process_backlog+0x97/0x2c0
__netif_receive_skb+0x22/0x80

Fixes: 0c71437dd50d ("can: j1939: j1939_session_deactivate(): clarify lifetime of session object")
Reported-by: syzbot+9981a614060dcee6eeca@syzkaller.appspotmail.com
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
---
net/can/j1939/transport.c | 4 ----
1 file changed, 4 deletions(-)

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index bdc95bd7a851..0f8309314075 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -1079,10 +1079,6 @@ static bool j1939_session_deactivate(struct j1939_session *session)
bool active;

j1939_session_list_lock(priv);
- /* This function should be called with a session ref-count of at
- * least 2.
- */
- WARN_ON_ONCE(kref_read(&session->kref) < 2);
active = j1939_session_deactivate_locked(session);
j1939_session_list_unlock(priv);

--
2.25.1