Re: [PATCH] can: j1939: prevent deadlock by changing j1939_socks_lock to rwlock

On Tue, 11 Jul 2023 17:47:50 -0700
Ziqi Zhao [off-list ref] wrote:

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.

#syz test:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

Signed-off-by: Ziqi Zhao <redacted>
---

Reader-writer locks are not the best way to fix a lock hierarchy problem.
Instead either fix the lock ordering, or use RCU.

Other devices don't have this problem, so perhaps the unique locking
in this device is the problem.