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

[Waiman Long]

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.

Cheers,
Longman