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

[Waiman Long <longman@redhat.com>]

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@redhat.com>
>>>>>> 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