Commit 87ca4f9e authored Dec 30, 2022 by Waiman Long Committed by Ingo Molnar Jan 09, 2023

sched/core: 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. 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 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.

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()")
Reported-by: David Wang 王标 <wangbiao3@xiaomi.com>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20221231041120.440785-2-longman@redhat.com

parent 7fb3ff22

kernel/sched/core.c

+29 −5

Original line number	Diff line number	Diff line
		@@ -2612,19 +2612,43 @@ void do_set_cpus_allowed(struct task_struct p, const struct cpumask new_mask)
		int dup_user_cpus_ptr(struct task_struct dst, struct task_struct src,
		int node)
		{
		cpumask_t *user_mask;
		unsigned long flags;

		if (!src->user_cpus_ptr)
		/*
		* Always clear dst->user_cpus_ptr first as their user_cpus_ptr's
		* may differ by now due to racing.
		*/
		dst->user_cpus_ptr = NULL;

		/*
		* This check is racy and losing the race is a valid situation.
		* It is not worth the extra overhead of taking the pi_lock on
		* every fork/clone.
		*/
		if (data_race(!src->user_cpus_ptr))
		return 0;

		dst->user_cpus_ptr = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
		if (!dst->user_cpus_ptr)
		user_mask = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
		if (!user_mask)
		return -ENOMEM;

		/* Use pi_lock to protect content of user_cpus_ptr */
		/*
		* Use pi_lock to protect content of user_cpus_ptr
		*
		* Though unlikely, user_cpus_ptr can be reset to NULL by a concurrent
		* do_set_cpus_allowed().
		*/
		raw_spin_lock_irqsave(&src->pi_lock, flags);
		if (src->user_cpus_ptr) {
		swap(dst->user_cpus_ptr, user_mask);
		cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
		}
		raw_spin_unlock_irqrestore(&src->pi_lock, flags);

		if (unlikely(user_mask))
		kfree(user_mask);

		return 0;
		}