On 2019/3/6 14:26, Mike Rapoport wrote: > Hi, > > On Wed, Mar 06, 2019 at 01:53:12PM +0800, zhong jiang wrote: >> On 2019/3/6 10:05, Andrea Arcangeli wrote: >>> Hello everyone, >>> >>> [ CC'ed Mike and Peter ] >>> >>> On Tue, Mar 05, 2019 at 02:42:00PM +0800, zhong jiang wrote: >>>> On 2019/3/5 14:26, Dmitry Vyukov wrote: >>>>> On Mon, Mar 4, 2019 at 4:32 PM zhong jiang <zhongjiang@xxxxxxxxxx> wrote: >>>>>> On 2019/3/4 22:11, Dmitry Vyukov wrote: >>>>>>> On Mon, Mar 4, 2019 at 3:00 PM zhong jiang <zhongjiang@xxxxxxxxxx> wrote: >>>>>>>> On 2019/3/4 15:40, Dmitry Vyukov wrote: >>>>>>>>> On Sun, Mar 3, 2019 at 5:19 PM zhong jiang <zhongjiang@xxxxxxxxxx> wrote: >>>>>>>>>> Hi, guys >>>>>>>>>> >>>>>>>>>> I also hit the following issue. but it fails to reproduce the issue by the log. >>>>>>>>>> >>>>>>>>>> it seems to the case that we access the mm->owner and deference it will result in the UAF. >>>>>>>>>> But it should not be possible that we specify the incomplete process to be the mm->owner. >>>>>>>>>> >>>>>>>>>> Any thoughts? >>>>>>>>> FWIW syzbot was able to reproduce this with this reproducer. >>>>>>>>> This looks like a very subtle race (threaded reproducer that runs >>>>>>>>> repeatedly in multiple processes), so most likely we are looking for >>>>>>>>> something like few instructions inconsistency window. >>>>>>>>> >>>>>>>> I has a little doubtful about the instrustions inconsistency window. >>>>>>>> >>>>>>>> I guess that you mean some smb barriers should be taken into account.:-) >>>>>>>> >>>>>>>> Because IMO, It should not be the lock case to result in the issue. >>>>>>> Since the crash was triggered on x86 _most likley_ this is not a >>>>>>> missed barrier. What I meant is that one thread needs to executed some >>>>>>> code, while another thread is stopped within few instructions. >>>>>>> >>>>>>> >>>>>> It is weird and I can not find any relationship you had said with the issue.:-( >>>>>> >>>>>> Because It is the cause that mm->owner has been freed, whereas we still deference it. >>>>>> >>>>>> From the lastest freed task call trace, It fails to create process. >>>>>> >>>>>> Am I miss something or I misunderstand your meaning. Please correct me. >>>>> Your analysis looks correct. I am just saying that the root cause of >>>>> this use-after-free seems to be a race condition. >>>>> >>>>> >>>>> >>>> Yep, Indeed, I can not figure out how the race works. I will dig up further. >>> Yes it's a race condition. >>> >>> We were aware about the non-cooperative fork userfaultfd feature >>> creating userfaultfd file descriptor that gets reported to the parent >>> uffd, despite they belong to mm created by failed forks. >>> >>> https://www.spinics.net/lists/linux-mm/msg136357.html >>> >> Hi, Andrea >> >> I still not clear why uffd ioctl can use the incomplete process as the mm->owner. >> and how to produce the race. > There is a C reproducer in the syzcaller report: > > https://syzkaller.appspot.com/x/repro.c?x=172fa5a3400000 > >> From your above explainations, My underdtanding is that the process handling do_exexve >> will have a temporary mm, which will be used by the UUFD ioctl. > The race is between userfaultfd operation and fork() failure: > > forking thread | userfaultfd monitor thread > --------------------------------+------------------------------- > fork() | > dup_mmap() | > dup_userfaultfd() | > dup_userfaultfd_complete() | > | read(UFFD_EVENT_FORK) > | uffdio_copy() > | mmget_not_zero() > goto bad_fork_something | > ... | > bad_fork_free: | > free_task() | > | mem_cgroup_from_task() > | /* access stale mm->owner */ > Hi, Mike forking thread fails to create the process ,and then free the allocated task struct. Other userfaultfd monitor thread should not access the stale mm->owner. The parent process and child process do not share the mm struct. Userfaultfd monitor thread's mm->owner should not point to the freed child task_struct. and due to the existence of tasklist_lock, we can not specify the mm->owner to freed task_struct. I miss something,=-O Thanks, zhong jiang >> Thanks, >> zhong jiang
