+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
