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Subject[PATCH v3] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()
Date
In general, a non-null user_cpus_ptr will remain set until the task dies.
A possible exception to this is the fact that do_set_cpus_allowed()
will clear a non-null user_cpus_ptr. To allow this possible racing
condition, we need to check for NULL user_cpus_ptr under the pi_lock
before duping the user mask.

Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/sched/core.c | 33 +++++++++++++++++++++++++++++----
1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8df51b08bb38..ee14aafeacba 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2624,19 +2624,44 @@ 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;

+ /*
+ * 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 (!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(). When this happens, we need to clear
+ * dst->user_cpus_ptr and free the allocated memory afterward.
+ */
raw_spin_lock_irqsave(&src->pi_lock, flags);
- cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+ 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;
}

--
2.31.1
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