linux-kernel - Re: [syzbot] possible deadlock in br_ioctl

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Message-ID: <CAK8P3a0dX1GRDTUp50kW81dD-dUw_=H4sx6tyeCVJea-FOBCQA@mail.gmail.com>
Date:   Mon, 2 Aug 2021 10:40:42 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Nikolay Aleksandrov <nikolay@...dia.com>
Cc:     Hillf Danton <hdanton@...a.com>,
        syzbot <syzbot+34fe5894623c4ab1b379@...kaller.appspotmail.com>,
        Arnd Bergmann <arnd@...db.de>,
        bridge@...ts.linux-foundation.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [syzbot] possible deadlock in br_ioctl_call

On Mon, Aug 2, 2021 at 10:30 AM Nikolay Aleksandrov <nikolay@...dia.com> wrote:
> On 01/08/2021 16:14, Hillf Danton wrote:
> > On Sun, 01 Aug 2021 03:34:24 -0700
> >> syzbot found the following issue on:
>
> Thanks, but it will need more work, the bridge ioctl calls were divided in two parts
> before: one was deviceless called by sock_ioctl and didn't expect rtnl to be held, the other was
> with a device called by dev_ifsioc() and expected rtnl to be held.
> Then ad2f99aedf8f ("net: bridge: move bridge ioctls out of .ndo_do_ioctl")
> united them in a single ioctl stub, but didn't take care of the locking expectations.
> For sock_ioctl now we acquire  (1) br_ioctl_mutex, (2) rtnl and for dev_ifsioc we
> acquire (1) rtnl, (2) br_ioctl_mutex as the lockdep warning has demonstrated.

Right, sorry about causing problems here.

> That fix above can work if rtnl gets reacquired by the ioctl in the proper switch cases.
> To avoid playing even more locking games it'd probably be best to always acquire and
> release rtnl by the bridge ioctl which will need a bit more work.
>
> Arnd, should I take care of it?

That would be best I think. As you have already analyzed the problem and come
up with a possible solution, I'm sure you will get to a better fix
more quickly than
I would.

Thanks,

       Arnd