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Date:   Wed, 6 Mar 2019 13:53:12 +0800
From:   zhong jiang <zhongjiang@...wei.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
CC:     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 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
>
> The fork failure in my testcase happened because of signal pending
> that interrupted fork after the failed-fork uffd context, was already
> pushed to the userfaultfd reader/monitor. CRIU then takes care of
> filtering the failed fork cases so we didn't want to make the fork
> code more complicated just for userfaultfd.
>
> In reality if MEMCG is enabled at build time, mm->owner maintainance
> code now creates a race condition in the above case, with any fork
> failure.
>
> I pinged Mike yesterday to ask if my theory could be true for this bug
> and one solution he suggested is to do the userfaultfd_dup at a point
> where fork cannot fail anymore. That's precisely what we were
> wondering to do back then to avoid the failed fork reports to the
> non cooperative uffd monitor.
>
> That will solve the false positive deliveries that CRIU manager
> currently filters out too. From a theoretical standpoint it's also
> quite strange to even allow any uffd ioctl to run on a otherwise long
> gone mm created for a process that in the end wasn't even created (the
> mm got temporarily fully created, but no child task really ever used
> such mm). However that mm is on its way to exit_mmap as soon as the
> ioclt returns and this only ever happens during race conditions, so
> the way CRIU monitor works there wasn't anything fundamentally
> concerning about this detail, despite it's remarkably "strange". Our
> priority was to keep the fork code as simple as possible and keep
> userfaultfd as non intrusive as possible.

Hi, Andrea

I still not clear why uffd ioctl can use the incomplete process as the mm->owner.
and how to produce the race.

>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.

Thanks,
zhong jiang
> One alternative solution I'm wondering about for this memcg issue is
> to free the task struct with RCU also when fork has failed and to add
> the mm_update_next_owner before mmput. That will still report failed
> forks to the uffd monitor, so it's not the ideal fix, but since it's
> probably simpler I'm posting it below. Also I couldn't reproduce the
> problem with the testcase here yet.
>
> >From 6cbf9d377b705476e5226704422357176f79e32c Mon Sep 17 00:00:00 2001
> From: Andrea Arcangeli <aarcange@...hat.com>
> Date: Tue, 5 Mar 2019 19:21:37 -0500
> Subject: [PATCH 1/1] userfaultfd: use RCU to free the task struct when fork
>  fails if MEMCG
>
> MEMCG depends on the task structure not to be freed under
> rcu_read_lock() in get_mem_cgroup_from_mm() after it dereferences
> mm->owner.
>
> A better fix would be to avoid registering forked vmas in userfaultfd
> contexts reported to the monitor, if case fork ends up failing.
>
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> ---
>  kernel/fork.c | 34 ++++++++++++++++++++++++++++++++--
>  1 file changed, 32 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index eb9953c82104..3bcbb361ffbc 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -953,6 +953,15 @@ static void mm_init_aio(struct mm_struct *mm)
>  #endif
>  }
>  
> +static __always_inline void mm_clear_owner(struct mm_struct *mm,
> +					   struct task_struct *p)
> +{
> +#ifdef CONFIG_MEMCG
> +	if (mm->owner == p)
> +		mm->owner = NULL;
> +#endif
> +}
> +
>  static void mm_init_owner(struct mm_struct *mm, struct task_struct *p)
>  {
>  #ifdef CONFIG_MEMCG
> @@ -1345,6 +1354,7 @@ static struct mm_struct *dup_mm(struct task_struct *tsk)
>  free_pt:
>  	/* don't put binfmt in mmput, we haven't got module yet */
>  	mm->binfmt = NULL;
> +	mm_init_owner(mm, NULL);
>  	mmput(mm);
>  
>  fail_nomem:
> @@ -1676,6 +1686,24 @@ static inline void rcu_copy_process(struct task_struct *p)
>  #endif /* #ifdef CONFIG_TASKS_RCU */
>  }
>  
> +#ifdef CONFIG_MEMCG
> +static void __delayed_free_task(struct rcu_head *rhp)
> +{
> +	struct task_struct *tsk = container_of(rhp, struct task_struct, rcu);
> +
> +	free_task(tsk);
> +}
> +#endif /* CONFIG_MEMCG */
> +
> +static __always_inline void delayed_free_task(struct task_struct *tsk)
> +{
> +#ifdef CONFIG_MEMCG
> +	call_rcu(&tsk->rcu, __delayed_free_task);
> +#else /* CONFIG_MEMCG */
> +	free_task(tsk);
> +#endif /* CONFIG_MEMCG */
> +}
> +
>  /*
>   * This creates a new process as a copy of the old one,
>   * but does not actually start it yet.
> @@ -2137,8 +2165,10 @@ static __latent_entropy struct task_struct *copy_process(
>  bad_fork_cleanup_namespaces:
>  	exit_task_namespaces(p);
>  bad_fork_cleanup_mm:
> -	if (p->mm)
> +	if (p->mm) {
> +		mm_clear_owner(p->mm, p);
>  		mmput(p->mm);
> +	}
>  bad_fork_cleanup_signal:
>  	if (!(clone_flags & CLONE_THREAD))
>  		free_signal_struct(p->signal);
> @@ -2169,7 +2199,7 @@ static __latent_entropy struct task_struct *copy_process(
>  bad_fork_free:
>  	p->state = TASK_DEAD;
>  	put_task_stack(p);
> -	free_task(p);
> +	delayed_free_task(p);
>  fork_out:
>  	spin_lock_irq(&current->sighand->siglock);
>  	hlist_del_init(&delayed.node);
>
> .
>


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