[PATCH 3/5] blk-cgroup: fix race between policy activation and blkg destruction

[Yu Kuai]

From: Zheng Qixing <zhengqixing@xxxxxxxxxx>

When switching an IO scheduler on a block device, blkcg_activate_policy()
allocates blkg_policy_data (pd) for all blkgs attached to the queue.
However, blkcg_activate_policy() may race with concurrent blkcg deletion,
leading to use-after-free and memory leak issues.

The use-after-free occurs in the following race:

T1 (blkcg_activate_policy):
  - Successfully allocates pd for blkg1 (loop0->queue, blkcgA)
  - Fails to allocate pd for blkg2 (loop0->queue, blkcgB)
  - Enters the enomem rollback path to release blkg1 resources

T2 (blkcg deletion):
  - blkcgA is deleted concurrently
  - blkg1 is freed via blkg_free_workfn()
  - blkg1->pd is freed

T1 (continued):
  - Rollback path accesses blkg1->pd->online after pd is freed
  - Triggers use-after-free

In addition, blkg_free_workfn() frees pd before removing the blkg from
q->blkg_list. This allows blkcg_activate_policy() to allocate a new pd
for a blkg that is being destroyed, leaving the newly allocated pd
unreachable when the blkg is finally freed.

Fix these races by extending blkcg_mutex coverage to serialize
blkcg_activate_policy() rollback and blkg destruction, ensuring pd
lifecycle is synchronized with blkg list visibility.

Fixes: f1c006f1c685 ("blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()")
Signed-off-by: Zheng Qixing <zhengqixing@xxxxxxxxxx>
Signed-off-by: Yu Kuai <yukuai@xxxxxxx>
---
 block/blk-cgroup.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index a98a22e06fd1..007dfc4f9c59 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1612,10 +1612,12 @@ int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol)
 	if (WARN_ON_ONCE(!pol->pd_alloc_fn || !pol->pd_free_fn))
 		return -EINVAL;
 
 	if (queue_is_mq(q))
 		memflags = blk_mq_freeze_queue(q);
+
+	mutex_lock(&q->blkcg_mutex);
 retry:
 	spin_lock_irq(&q->queue_lock);
 
 	/* blkg_list is pushed at the head, reverse walk to initialize parents first */
 	list_for_each_entry_reverse(blkg, &q->blkg_list, q_node) {
@@ -1674,10 +1676,11 @@ int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol)
 	__set_bit(pol->plid, q->blkcg_pols);
 	ret = 0;
 
 	spin_unlock_irq(&q->queue_lock);
 out:
+	mutex_unlock(&q->blkcg_mutex);
 	if (queue_is_mq(q))
 		blk_mq_unfreeze_queue(q, memflags);
 	if (pinned_blkg)
 		blkg_put(pinned_blkg);
 	if (pd_prealloc)
-- 
2.51.0