Re: [PATCH-tip v2] sched: Fix use-after-free bug in dup_user_cpus_ptr()

[Waiman Long <longman@redhat.com>]

On 12/22/22 14:34, Peter Zijlstra wrote:
> On Mon, Dec 05, 2022 at 11:48:32AM -0500, Waiman Long wrote:
>> Since commit 07ec77a1d4e8 ("sched: Allow task CPU affinity to be
>> restricted on asymmetric systems"), the setting and clearing of
>> user_cpus_ptr are done under pi_lock for arm64 architecture. However,
>> dup_user_cpus_ptr() accesses user_cpus_ptr without any lock
>> protection. When racing with the clearing of user_cpus_ptr in
>> __set_cpus_allowed_ptr_locked(), it can lead to user-after-free and
>> double-free in arm64 kernel.
> How? the task cannot be in migrate_enable() and fork() at the same time,
> no?
>
I believe a task A can call sched_setaffinity() to modify the cpu 
affinity of a different task, say B, which can be under fork() at the 
same time. So we need to use the pi_lock to synchronize the access of 
user_cpus_ptr to avoid the kind of race that can cause double-free.

Cheers,
Longman