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

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Date:   Fri, 15 Dec 2017 08:35:05 +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 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.



>> 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);
>>
>>       /*