On Fri, Dec 15, 2017 at 12:00 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > On Fri, Dec 15, 2017 at 8:35 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: >> On Fri, Dec 15, 2017 at 7:56 AM, Eric W. Biederman >> <ebiederm@xxxxxxxxxxxx> wrote: >>> Cong Wang <xiyou.wangcong@xxxxxxxxx> writes: >>> >>>> syzbot reported we have a use-after-free when mqueue_evict_inode() >>>> is called on __cleanup_mnt() path, where the ipc ns is already >>>> freed by the previous exit_task_namespaces(). We can just move >>>> it after after exit_task_work() to avoid this use-after-free. >>> >>> How does that possibly work. (I haven't seen this syzbot report). >>> >>> Looking at the code we have get_ns_from_inode. Which takes the mq_lock, >>> sees if the pointer is NULL and takes a reference if it is non-NULL. >>> >>> Meanwhile put_ipc_ns calls mq_clear_sbinfo(ns) with the mq_lock held >>> when the count drops to zero. >>> >>> Where is the race in that? >>> >>> The rest of mqueue_evict_inode uses the returned pointer and >>> tests that the pointer is non-NULL before user it. >>> >>> So either szbot is giving you a bad report or there is a subtle race >>> there I am not seeing. The change below is not at all the proper way to >>> fix a subtle race. >>> >>> Eric >> >> Cong, what was that report? Searching by >> "exit_task_work|exit_task_namespaces" there are too many of them: >> https://groups.google.com/forum/#!searchin/syzkaller-bugs/%22exit_task_work$7Cexit_task_namespaces%22%7Csort:date >> >> I can only say that syzbot does not make up reports. That's something >> that actually happened and was provoked by userspace. > > > Ah, found that bug: > https://groups.google.com/d/msg/syzkaller-bugs/1XBaqnPSXzs/VF-eCSPuCQAJ Yeah, and it is introduced by: http://git.cmpxchg.org/cgit.cgi/linux-mmots.git/commit/?id=9c583773d036336176e9e50441890659bc4eeae8