linux-kernel - Re: [PATCH] can: j1939: prevent deadlock by changing j1939_socks

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Date:   Thu, 13 Jul 2023 15:23:05 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Ziqi Zhao <astrajoan@...oo.com>
Cc:     syzbot+1591462f226d9cbf0564@...kaller.appspotmail.com,
        davem@...emloft.net, dvyukov@...gle.com, edumazet@...gle.com,
        ivan.orlov0322@...il.com, kernel@...gutronix.de, kuba@...nel.org,
        linux-can@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux@...pel-privat.de, mkl@...gutronix.de, netdev@...r.kernel.org,
        o.rempel@...gutronix.de, pabeni@...hat.com, robin@...tonic.nl,
        skhan@...uxfoundation.org, socketcan@...tkopp.net,
        syzkaller-bugs@...glegroups.com, syzkaller@...glegroups.com
Subject: Re: [PATCH] can: j1939: prevent deadlock by changing
 j1939_socks_lock to rwlock

On Tue, 11 Jul 2023 17:47:50 -0700
Ziqi Zhao <astrajoan@...oo.com> 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 <astrajoan@...oo.com>
> ---

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.