On Sat, Dec 16, 2017 at 1:00 AM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote: >>>>> 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 If it's going to be discussed here, note that it was reported by syzbot and requires "#syz fix" tag after fixing here: https://groups.google.com/d/msg/syzkaller-bugs/1XBaqnPSXzs/jGDCMoz_AQAJ