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

commit 87ca4f9efbd7cc649ff43b87970888f2812945b8 upstream.

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()")
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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bb684fe1b96ed4af6ac8291f2137452b6089bdef..2bd5e235d078141cee10757c1b794f9a4ccc2c75 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2501,14 +2501,43 @@ 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)
 {
-       if (!src->user_cpus_ptr)
+       cpumask_t *user_mask;
+       unsigned long flags;
+
+       /*
+        * 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;
 
-       cpumask_copy(dst->user_cpus_ptr, src->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;
 }