From: Zheng Qixing 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 Reviewed-by: Christoph Hellwig --- 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