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Date:   Wed, 6 Mar 2019 08:26:25 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     zhong jiang <zhongjiang@...wei.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+cbb52e396df3e565ab02@...kaller.appspotmail.com>,
        Michal Hocko <mhocko@...nel.org>, cgroups@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        David Rientjes <rientjes@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Peter Xu <peterx@...hat.com>
Subject: Re: KASAN: use-after-free Read in get_mem_cgroup_from_mm

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@...wei.com> wrote:
> >>>> On 2019/3/4 22:11, Dmitry Vyukov wrote:
> >>>>> On Mon, Mar 4, 2019 at 3:00 PM zhong jiang <zhongjiang@...wei.com> wrote:
> >>>>>> On 2019/3/4 15:40, Dmitry Vyukov wrote:
> >>>>>>> On Sun, Mar 3, 2019 at 5:19 PM zhong jiang <zhongjiang@...wei.com> 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 */
 
> Thanks,
> zhong jiang

-- 
Sincerely yours,
Mike.

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