linux-kernel - Re: [PATCH] rfkill: fix deadlock in rfkill_send

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Message-ID: <20231013110638.GD29570@kernel.org>
Date:   Fri, 13 Oct 2023 13:06:38 +0200
From:   Simon Horman <horms@...nel.org>
To:     Edward AD <twuufnxlz@...il.com>
Cc:     syzbot+509238e523e032442b80@...kaller.appspotmail.com,
        davem@...emloft.net, edumazet@...gle.com, johannes.berg@...el.com,
        johannes@...solutions.net, kuba@...nel.org,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, pabeni@...hat.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] rfkill: fix deadlock in rfkill_send_events

On Tue, Oct 10, 2023 at 09:08:15AM +0800, Edward AD wrote:
> syzbot report:
> syz-executor675/5132 is trying to acquire lock:
> ffff8880297ee088 (&data->mtx){+.+.}-{3:3}, at: rfkill_send_events+0x226/0x3f0 net/rfkill/core.c:286
> 
> but task is already holding lock:
> ffff88801bfc0088 (&data->mtx){+.+.}-{3:3}, at: rfkill_fop_open+0x146/0x750 net/rfkill/core.c:1183
> 
> other info that might help us debug this:
>  Possible unsafe locking scenario:
> 
>        CPU0
>        ----
>   lock(&data->mtx);
>   lock(&data->mtx);
> 
>  *** DEADLOCK ***
> 
> In 2c3dfba4cf84 insert rfkill_sync() to rfkill_fop_open(), it will call
> rfkill_send_events() and then triger this issue.
> 
> Fixes: 2c3dfba4cf84 ("rfkill: sync before userspace visibility/changes")
> Reported-and-tested-by: syzbot+509238e523e032442b80@...kaller.appspotmail.com
> Signed-off-by: Edward AD <twuufnxlz@...il.com>

Hi Edward,

I am wondering if you considered moving the rfkill_sync() calls
to before &data->mtx is taken, to avoid the need to drop and
retake it?

Perhaps it doesn't work for some reason (compile tested only!).
But this does seem somehow cleaner for me.