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Date:   Fri, 17 Nov 2023 09:10:02 +0100
From:   Oleksij Rempel <o.rempel@pengutronix.de>
To:     Ziqi Zhao <astrajoan@yahoo.com>
Cc:     ivan.orlov0322@gmail.com, edumazet@google.com,
        syzbot+881d65229ca4f9ae8c84@syzkaller.appspotmail.com,
        socketcan@hartkopp.net, bridge@lists.linux-foundation.org,
        nikolay@nvidia.com,
        syzbot+1591462f226d9cbf0564@syzkaller.appspotmail.com,
        roopa@nvidia.com, kuba@kernel.org, pabeni@redhat.com,
        arnd@arndb.de, syzkaller-bugs@googlegroups.com,
        mudongliangabcd@gmail.com, linux-can@vger.kernel.org,
        mkl@pengutronix.de, skhan@linuxfoundation.org, robin@protonic.nl,
        linux-kernel@vger.kernel.org, linux@rempel-privat.de,
        kernel@pengutronix.de, netdev@vger.kernel.org, davem@davemloft.net
Subject: Re: [PATCH] can: j1939: prevent deadlock by changing
 j1939_socks_lock to rwlock
Message-ID: <20231117081002.GA590719@pengutronix.de>
References: <20230704064710.3189-1-astrajoan@yahoo.com>
 <20230721162226.8639-1-astrajoan@yahoo.com>
 <20230807044634.GA5736@pengutronix.de>
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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:
> > 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 <astrajoan@yahoo.com>

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

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