linux-kernel - [PATCH v2 2/3] blk-cgroup: skip dying blkg in blkcg_activate

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Message-Id: <20260113061035.1902522-3-zhengqixing@huaweicloud.com>
Date: Tue, 13 Jan 2026 14:10:34 +0800
From: Zheng Qixing <zhengqixing@...weicloud.com>
To: tj@...nel.org,
	josef@...icpanda.com,
	axboe@...nel.dk,
	yukuai3@...wei.com,
	hch@...radead.org
Cc: cgroups@...r.kernel.org,
	linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	mkoutny@...e.com,
	yi.zhang@...wei.com,
	yangerkun@...wei.com,
	houtao1@...wei.com,
	zhengqixing@...wei.com
Subject: [PATCH v2 2/3] blk-cgroup: skip dying blkg in blkcg_activate_policy()

From: Zheng Qixing <zhengqixing@...wei.com>

When switching IO schedulers on a block device, blkcg_activate_policy()
can race with concurrent blkcg deletion, leading to a use-after-free in
rcu_accelerate_cbs.

T1:                               T2:
		                  blkg_destroy
                 		  kill(&blkg->refcnt) // blkg->refcnt=1->0
				  blkg_release // call_rcu(__blkg_release)
                                  ...
				  blkg_free_workfn
                                  ->pd_free_fn(pd)
elv_iosched_store
elevator_switch
...
iterate blkg list
blkg_get(blkg) // blkg->refcnt=0->1
                                  list_del_init(&blkg->q_node)
blkg_put(pinned_blkg) // blkg->refcnt=1->0
blkg_release // call_rcu again
rcu_accelerate_cbs // uaf

Fix this by replacing blkg_get() with blkg_tryget(), which fails if
the blkg's refcount has already reached zero. If blkg_tryget() fails,
skip processing this blkg since it's already being destroyed.

Link: https://lore.kernel.org/all/20260108014416.3656493-4-zhengqixing@huaweicloud.com/
Fixes: f1c006f1c685 ("blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()")
Signed-off-by: Zheng Qixing <zhengqixing@...wei.com>
Reviewed-by: Christoph Hellwig <hch@....de>
---
 block/blk-cgroup.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 600f8c5843ea..5dbc107eec53 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1622,9 +1622,10 @@ int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol)
 			 * GFP_NOWAIT failed.  Free the existing one and
 			 * prealloc for @blkg w/ GFP_KERNEL.
 			 */
+			if (!blkg_tryget(blkg))
+				continue;
 			if (pinned_blkg)
 				blkg_put(pinned_blkg);
-			blkg_get(blkg);
 			pinned_blkg = blkg;
 
 			spin_unlock_irq(&q->queue_lock);
-- 
2.39.2