[PATCH v2 1/4] blk-cgroup: protect q->blkg_list iteration in blkg_destroy_all() with blkcg_mutex

[Yu Kuai <yukuai@kernel.org>]

From: Yu Kuai <yukuai@fygo.io>

blkg_destroy_all() iterates q->blkg_list without holding blkcg_mutex,
which can race with blkg_free_workfn() that removes blkgs from the list
while holding blkcg_mutex.

Add blkcg_mutex protection around the q->blkg_list iteration to prevent
potential list corruption or use-after-free issues.

Reviewed-by: Tang Yizhou <yizhou.tang@shopee.com>
Signed-off-by: Yu Kuai <yukuai@fygo.io>
---
 block/blk-cgroup.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index ee076ab795d3..7baccfb690fe 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -574,10 +574,11 @@ static void blkg_destroy_all(struct gendisk *disk)
 	struct blkcg_gq *blkg;
 	int count = BLKG_DESTROY_BATCH_SIZE;
 	int i;
 
 restart:
+	mutex_lock(&q->blkcg_mutex);
 	spin_lock_irq(&q->queue_lock);
 	list_for_each_entry(blkg, &q->blkg_list, q_node) {
 		struct blkcg *blkcg = blkg->blkcg;
 
 		if (hlist_unhashed(&blkg->blkcg_node))
@@ -592,10 +593,11 @@ static void blkg_destroy_all(struct gendisk *disk)
 		 * it when a batch of blkgs are destroyed.
 		 */
 		if (!(--count)) {
 			count = BLKG_DESTROY_BATCH_SIZE;
 			spin_unlock_irq(&q->queue_lock);
+			mutex_unlock(&q->blkcg_mutex);
 			cond_resched();
 			goto restart;
 		}
 	}
 
@@ -611,10 +613,11 @@ static void blkg_destroy_all(struct gendisk *disk)
 			__clear_bit(pol->plid, q->blkcg_pols);
 	}
 
 	q->root_blkg = NULL;
 	spin_unlock_irq(&q->queue_lock);
+	mutex_unlock(&q->blkcg_mutex);
 
 	wake_up_var(&q->root_blkg);
 }
 
 static void blkg_iostat_set(struct blkg_iostat *dst, struct blkg_iostat *src)
-- 
2.51.0