linux-kernel - Re: [PATCH] exit: move exit_task_namespaces() after exit_task

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Date:   Fri, 15 Dec 2017 09:00:38 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] exit: move exit_task_namespaces() after exit_task_work()

On Fri, Dec 15, 2017 at 8:35 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Fri, Dec 15, 2017 at 7:56 AM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> Cong Wang <xiyou.wangcong@...il.com> 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




>>> Reported-by: syzbot <syzkaller@...glegroups.com>
>>> Cc: Ingo Molnar <mingo@...nel.org>
>>> Cc: Al Viro <viro@...iv.linux.org.uk>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>>> Cc: stable@...r.kernel.org
>>> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
>>> ---
>>>  kernel/exit.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/exit.c b/kernel/exit.c
>>> index 6b4298a41167..909e43c45158 100644
>>> --- a/kernel/exit.c
>>> +++ b/kernel/exit.c
>>> @@ -861,8 +861,8 @@ void __noreturn do_exit(long code)
>>>       exit_fs(tsk);
>>>       if (group_dead)
>>>               disassociate_ctty(1);
>>> -     exit_task_namespaces(tsk);
>>>       exit_task_work(tsk);
>>> +     exit_task_namespaces(tsk);
>>>       exit_thread(tsk);
>>>
>>>       /*