CVE-2021-4454

In the Linux kernel, the following vulnerability has been resolved:

can: j1939: fix errant WARN_ON_ONCE in j1939_session_deactivate

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