Hi Desmond, On Mon, Aug 29, 2022 at 11:48 PM Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx> wrote: > > +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? We probably need to introduce a test to rfcomm-tester to reproduce this sort of problem, I also would like to avoid introducing a work just to trigger a state change since we don't have such problem on the likes of L2CAP socket so perhaps we need to rework the code a little bit to avoid the locking problems. > Best, > Desmond -- Luiz Augusto von Dentz
