Re: [PATCH v6 1/2] sched: Fix use-after-free bug in dup_user_cpus_ptr()

[Will Deacon]

On Fri, Dec 30, 2022 at 11:11:19PM -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. Since sched_setaffinity() can be invoked from another
> process, the process being modified may be undergoing fork() at
> the same time.  When racing with the clearing of user_cpus_ptr in
> __set_cpus_allowed_ptr_locked(), it can lead to user-after-free and
> possibly double-free in arm64 kernel.
> 
> Commit 8f9ea86fdf99 ("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 851a723e45d1 ("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.
> 
> Fixes: 07ec77a1d4e8 ("sched: Allow task CPU affinity to be restricted on asymmetric systems")
> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
> CC: stable@xxxxxxxxxxxxxxx
> Reported-by: David Wang 王标 <wangbiao3@xxxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  kernel/sched/core.c | 34 +++++++++++++++++++++++++++++-----
>  1 file changed, 29 insertions(+), 5 deletions(-)

Acked-by: Will Deacon <will@xxxxxxxxxx>

Will