RE: [External Mail]Re: [PATCH-tip v4] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()

[Wenjie Li (Evan)]

Dear Will and Longman.

Maybe we can solve this scenario firstly if it dose not effect other architecture.

Thanks.

-----Original Message-----
From: David Wang 王标 <wangbiao3@xxxxxxxxxx> 
Sent: Wednesday, November 30, 2022 7:52 PM
To: Waiman Long <longman@xxxxxxxxxx>; Will Deacon <will@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Juri Lelli <juri.lelli@xxxxxxxxxx>; Vincent Guittot <vincent.guittot@xxxxxxxxxx>; Dietmar Eggemann <dietmar.eggemann@xxxxxxx>; Steven Rostedt <rostedt@xxxxxxxxxxx>; Ben Segall <bsegall@xxxxxxxxxx>; Mel Gorman <mgorman@xxxxxxx>; Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>; Phil Auld <pauld@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Wenjie Li (QUIC) <quic_wenjieli@xxxxxxxxxxx>; chenhaihua@xxxxxxxxxx; yangjianlong@xxxxxxxxxx; 刘添威 <liutianwei@xxxxxxxxxx>; mazhenhua@xxxxxxxxxx; Ting11 Wang 王婷 <wangting11@xxxxxxxxxx>
Subject: 答复: [External Mail]Re: [PATCH-tip v4] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()

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Dear Will,  Longman,

Could we fix the issue first we met?  We can analyze other issue later.

Thanks


-----邮件原件-----
发件人: Waiman Long <longman@xxxxxxxxxx>
发送时间: 2022年11月30日 0:04
收件人: Will Deacon <will@xxxxxxxxxx>
抄送: Ingo Molnar <mingo@xxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Juri Lelli <juri.lelli@xxxxxxxxxx>; Vincent Guittot <vincent.guittot@xxxxxxxxxx>; Dietmar Eggemann <dietmar.eggemann@xxxxxxx>; Steven Rostedt <rostedt@xxxxxxxxxxx>; Ben Segall <bsegall@xxxxxxxxxx>; Mel Gorman <mgorman@xxxxxxx>; Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>; Phil Auld <pauld@xxxxxxxxxx>; Wenjie Li <wenjieli@xxxxxxxxxxxxxxxx>; David Wang 王标 <wangbiao3@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
主题: [External Mail]Re: [PATCH-tip v4] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()

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On 11/29/22 10:57, Will Deacon wrote:
> On Tue, Nov 29, 2022 at 10:32:49AM -0500, Waiman Long wrote:
>> On 11/29/22 09:07, Will Deacon wrote:
>>> On Mon, Nov 28, 2022 at 10:11:52AM -0500, Waiman Long wrote:
>>>> On 11/28/22 07:00, Will Deacon wrote:
>>>>> On Sun, Nov 27, 2022 at 08:43:27PM -0500, Waiman Long wrote:
>>>>>> On 11/24/22 21:39, Waiman Long wrote:
>>>>>>> In general, a non-null user_cpus_ptr will remain set until the task dies.
>>>>>>> A possible exception to this is the fact that
>>>>>>> do_set_cpus_allowed() will clear a non-null user_cpus_ptr. To 
>>>>>>> allow this possible racing condition, we need to check for NULL 
>>>>>>> user_cpus_ptr under the pi_lock before duping the user mask.
>>>>>>>
>>>>>>> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in
>>>>>>> do_set_cpus_allowed()")
>>>>>>> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
>>>>>> This is actually a pre-existing use-after-free bug since commit
>>>>>> 07ec77a1d4e8
>>>>>> ("sched: Allow task CPU affinity to be restricted on asymmetric systems").
>>>>>> So it needs to be fixed in the stable release as well. Will 
>>>>>> resend the patch with an additional fixes tag and updated commit log.
>>>>> Please can you elaborate on the use-after-free here? Looking at 
>>>>> 07ec77a1d4e8, the mask is only freed in free_task() when the usage 
>>>>> refcount has dropped to zero and I can't see how that can race with fork().
>>>>>
>>>>> What am I missing?
>>>> I missed that at first. The current task cloning process copies the 
>>>> content of the task structure over to the newly cloned/forked task.
>>>> IOW, if user_cpus_ptr had been set up previously, it will be copied 
>>>> over to the cloned task. Now if user_cpus_ptr of the source task is 
>>>> cleared right after that and before dup_user_cpus_ptr() is called.
>>>> The obsolete user_cpus_ptr value in the cloned task will remain and get used even if it has been freed.
>>>> That is what I call as use-after-free and double-free.
>>> If the parent task can be modified concurrently with
>>> dup_task_struct() then surely we'd have bigger issues because that's 
>>> not going to be atomic? At the very least we'd have a data race, but 
>>> it also feels like we could end up with inconsistent task state in 
>>> the child. In fact, couldn't the normal 'cpus_mask' be corrupted by a concurrent set_cpus_allowed_common()?
>>>
>>> Or am I still failing to understand the race?
>>>
>> A major difference between cpus_mask and user_cpus_ptr is that for 
>> cpus_mask, the bitmap is embedded into task_struct whereas 
>> user_cpus_ptr is a pointer to an external bitmap. So there is no 
>> issue of use-after-free wrt cpus_mask. That is not the case where the 
>> memory of the user_cpus_ptr of the parent task is freed, but then a 
>> reference to that memory is still available in the child's task struct and may be used.
> Sure, I'm not saying there's a UAF on cpus_mask, but I'm concerned 
> that we could corrupt the data and end up with an affinity mask that 
> doesn't correspond to anything meaningful. Do you agree that's possible?
That is certainly possible. So we have to be careful about it.
>
>> Note that the problematic concurrence is not between the copying of 
>> task struct and changing of the task struct. It is what will happen 
>> after the task struct copying has already been done with an extra 
>> reference present in the child's task struct.
> Well, sort of, but the child only has the extra reference _because_ 
> the parent pointer was concurrently cleared to NULL, otherwise
> dup_user_cpus_ptr() would have allocated a new copy and we'd be ok, no?
Yes, that is exactly where the problem is and this is what my patch is trying to fix.
>
> Overall, I'm just very wary that we seem to be saying that
> copy_process() can run concurrently with changes to the parent. Maybe 
> it's all been written with that in mindi (including all the arch 
> callbacks), but I'd be astonished if this is the only problem in there.

It seems like that, at least in some cases, the clearing of a task's user_cpus_ptr can be done by another task. So the parent may be unaware of it and so is not its fault.

Cheers,
Longman

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