Hi Desmond, On Tue, Aug 30, 2022 at 10:41 AM Luiz Augusto von Dentz <luiz.dentz@xxxxxxxxx> wrote: > > 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. It looks like for L2CAP we use lock_sock_nested on teardown, we don't have the exact same behavior in RFCOMM but I think that might be worth a try if we can use that instead of introducing yet another work item. > > Best, > > Desmond > > > > -- > Luiz Augusto von Dentz -- Luiz Augusto von Dentz
