FROMLIST: sched: Fix use-after-free bug in dup_user_cpus_ptr()

Since commit 07ec77a1 ("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.

Commit 8f9ea86f ("sched: Always preserve the user requested
cpumask") fixes this problem as user_cpus_ptr, once set, will never
be cleared in a task's lifetime. However, this bug was re-introduced
in commit 851a723e ("sched: Always clear user_cpus_ptr in
do_set_cpus_allowed()") which allows the clearing of user_cpus_ptr in
do_set_cpus_allowed(). This time, it will affect all arches.

Fix this bug by always clearing the user_cpus_ptr of the newly
cloned/forked task before the copying process starts and check the
user_cpus_ptr state of the source task under pi_lock.

Note to stable, this patch won't be applicable to stable releases.
Just copy the new dup_user_cpus_ptr() function over.

Bug: 256578302
Link: https://lore.kernel.org/all/20221128014441.1264867-1-longman@redhat.com/


Fixes: 07ec77a1 ("sched: Allow task CPU affinity to be restricted on asymmetric systems")
Fixes: 851a723e ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
CC: stable@vger.kernel.org
Reported-by: David Wang 王标 <wangbiao3@xiaomi.com>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: biao wang <wangbiao3@xiaomi.com>
Signed-off-by: wangbiao3 <wangbiao3@xiaomi.corp-partner.google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: Ib5d2b0632ba205a65455465ee9d83db7c29e9111