Messages in this thread | | | Date | Tue, 29 Nov 2022 14:07:59 +0000 | From | Will Deacon <> | Subject | Re: [PATCH-tip v4] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr() |
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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?
Will
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