workqueue: Put the pwq after detaching the rescuer from the pool

The commit 68f83057b913("workqueue: Reap workers via kthread_stop() and
remove detach_completion") adds code to reap the normal workers but
mistakenly does not handle the rescuer and also removes the code waiting
for the rescuer in put_unbound_pool(), which caused a use-after-free bug
reported by Cheung Wall.

To avoid the use-after-free bug, the pool’s reference must be held until
the detachment is complete. Therefore, move the code that puts the pwq
after detaching the rescuer from the pool.

Reported-by: cheung wall <zzqq0103.hey@gmail.com>
Cc: cheung wall <zzqq0103.hey@gmail.com>
Link: https://lore.kernel.org/lkml/CAKHoSAvP3iQW+GwmKzWjEAOoPvzeWeoMO0Gz7Pp3_4kxt-RMoA@mail.gmail.com/
Fixes: 68f83057b913("workqueue: Reap workers via kthread_stop() and remove detach_completion")
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 33a23c7b22747c9104b4ea30c930345daa78c51e..ccad33001c58cd52b3671a723f505f558123f678 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3516,12 +3516,6 @@ repeat:
                        }
                }
 
-               /*
-                * Put the reference grabbed by send_mayday().  @pool won't
-                * go away while we're still attached to it.
-                */
-               put_pwq(pwq);
-
                /*
                 * Leave this pool. Notify regular workers; otherwise, we end up
                 * with 0 concurrency and stalling the execution.
@@ -3532,6 +3526,12 @@ repeat:
 
                worker_detach_from_pool(rescuer);
 
+               /*
+                * Put the reference grabbed by send_mayday().  @pool might
+                * go away any time after it.
+                */
+               put_pwq_unlocked(pwq);
+
                raw_spin_lock_irq(&wq_mayday_lock);
        }