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Date:   Wed, 6 Mar 2019 15:41:06 +0800
From:   zhong jiang <zhongjiang@...wei.com>
To:     Mike Rapoport <rppt@...ux.ibm.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

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@...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 */
>  
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


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