sched_setaffinity() can also race with a concurrent fork/clone() syscall
calling dup_user_cpus_ptr(). That may lead to a use after free problem.
Fix that by protecting the cpumask copying using pi_lock of the source
task.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/sched/core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1ba5d82d4f3c..89c393df5ebc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2612,6 +2612,8 @@ 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)
 {
+	unsigned long flags;
+
 	if (!src->user_cpus_ptr)
 		return 0;
 
@@ -2619,7 +2621,10 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
 	if (!dst->user_cpus_ptr)
 		return -ENOMEM;
 
+	/* Use pi_lock to protect content of user_cpus_ptr */
+	raw_spin_lock_irqsave(&src->pi_lock, flags);
 	cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+	raw_spin_unlock_irqrestore(&src->pi_lock, flags);
 	return 0;
 }
 
-- 
2.31.1