Messages in this thread | | | Date | Tue, 29 Nov 2022 11:03:35 -0500 | Subject | Re: [PATCH-tip v4] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr() | From | Waiman Long <> |
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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@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
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