On Mon, Aug 07, 2023 at 06:46:34AM +0200, Oleksij Rempel wrote:

On Fri, Jul 21, 2023 at 09:22:26AM -0700, Ziqi Zhao wrote:

quoted

The following 3 locks would race against each other, causing the
deadlock situation in the Syzbot bug report:

- j1939_socks_lock
- active_session_list_lock
- sk_session_queue_lock

A reasonable fix is to change j1939_socks_lock to an rwlock, since in
the rare situations where a write lock is required for the linked list
that j1939_socks_lock is protecting, the code does not attempt to
acquire any more locks. This would break the circular lock dependency,
where, for example, the current thread already locks j1939_socks_lock
and attempts to acquire sk_session_queue_lock, and at the same time,
another thread attempts to acquire j1939_socks_lock while holding
sk_session_queue_lock.

NOTE: This patch along does not fix the unregister_netdevice bug
reported by Syzbot; instead, it solves a deadlock situation to prepare
for one or more further patches to actually fix the Syzbot bug, which
appears to be a reference counting problem within the j1939 codebase.

Reported-by: syzbot+1591462f226d9cbf0564@syzkaller.appspotmail.com
Signed-off-by: Ziqi Zhao <redacted>

Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>

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