[PATCH v3 4/7] blk-cgroup: skip dying blkg in blkcg_activate_policy()

[Yu Kuai <yukuai@fnnas.com>]

From: Zheng Qixing <zhengqixing@huawei.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 checking hlist_unhashed(&blkg->blkcg_node) before getting
a reference to the blkg. This is the same check used in blkg_destroy()
to detect if a blkg has already been destroyed. If the blkg is already
unhashed, skip processing it since it's 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@huawei.com>
Signed-off-by: Yu Kuai <yukuai@fnnas.com>
---
 block/blk-cgroup.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index a6ac6ba9430d..f5b14a1d6973 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1610,6 +1610,8 @@ int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol)
 
 		if (blkg->pd[pol->plid])
 			continue;
+		if (hlist_unhashed(&blkg->blkcg_node))
+			continue;
 
 		/* If prealloc matches, use it; otherwise try GFP_NOWAIT */
 		if (blkg == pinned_blkg) {
-- 
2.51.0