linux-kernel - [PATCH] workqueue: Put the pwq after detaching the rescuer from the pool

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Message-Id: <20250123082535.1538074-1-jiangshanlai@gmail.com>
Date: Thu, 23 Jan 2025 16:25:35 +0800
From: Lai Jiangshan <jiangshanlai@...il.com>
To: linux-kernel@...r.kernel.org
Cc: Lai Jiangshan <jiangshan.ljs@...group.com>,
	cheung wall <zzqq0103.hey@...il.com>,
	Tejun Heo <tj@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>
Subject: [PATCH] workqueue: Put the pwq after detaching the rescuer from the pool

From: Lai Jiangshan <jiangshan.ljs@...group.com>

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@...il.com>
Cc: cheung wall <zzqq0103.hey@...il.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@...group.com>
---
 kernel/workqueue.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 33a23c7b2274..ccad33001c58 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3516,12 +3516,6 @@ static int rescuer_thread(void *__rescuer)
 			}
 		}
 
-		/*
-		 * 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 @@ static int rescuer_thread(void *__rescuer)
 
 		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);
 	}
 
-- 
2.19.1.6.gb485710b