Re: possible deadlock in rfcomm_sk_state_change

[Desmond Cheong Zhi Xi]

+cc Bluetooth and Networking maintainers

Hi Jiacheng,

On 28/8/22 04:03, Jiacheng Xu wrote:
> Hi,
>
> I believe the deadlock is more than possible but actually real.
> I got a poc that could stably trigger the deadlock.
>
> poc: https://drive.google.com/file/d/1PjqvMtHsrrGM1MIRGKl_zJGR-teAMMQy/view?usp=sharing
>
> Description/Root cause:
> In rfcomm_sock_shutdown(), lock_sock() is called when releasing and
> shutting down socket.
> However, lock_sock() has to be called once more when the sk_state is
> changed because the
> lock is not always held when rfcomm_sk_state_change() is called. One
> such call stack is:
>
>    rfcomm_sock_shutdown():
>      lock_sock();
>      __rfcomm_sock_close():
>        rfcomm_dlc_close():
>          __rfcomm_dlc_close():
>            rfcomm_dlc_lock();
>            rfcomm_sk_state_change():
>              lock_sock();
>
> Besides the recursive deadlock, there is also an
> issue of a lock hierarchy inversion between rfcomm_dlc_lock() and
> lock_sock() if the socket is locked in rfcomm_sk_state_change().


Thanks for the poc and for following the trail all the way to the root 
cause - this was a known issue and I didn't realize the patch wasn't 
applied.

>  > Reference: 
https://lore.kernel.org/all/20211004180734.434511-1-desmondcheongzx@xxxxxxxxx/
>

Fwiw, I tested the patch again with syzbot. It still applies cleanly to 
the head of bluetooth-next and seems to address the root cause.

Any thoughts from the maintainers on this issue and the proposed fix?

Best,
Desmond