From 89e1fb7ceffd898505ad7fa57acec0585bfaa2cc Mon Sep 17 00:00:00 2001 From: Fengnan Chang Date: Fri, 28 Nov 2025 16:53:14 +0800 Subject: blk-mq: fix potential uaf for 'queue_hw_ctx' This is just apply Kuai's patch in [1] with mirror changes. blk_mq_realloc_hw_ctxs() will free the 'queue_hw_ctx'(e.g. undate submit_queues through configfs for null_blk), while it might still be used from other context(e.g. switch elevator to none): t1 t2 elevator_switch blk_mq_unquiesce_queue blk_mq_run_hw_queues queue_for_each_hw_ctx // assembly code for hctx = (q)->queue_hw_ctx[i] mov 0x48(%rbp),%rdx -> read old queue_hw_ctx __blk_mq_update_nr_hw_queues blk_mq_realloc_hw_ctxs hctxs = q->queue_hw_ctx q->queue_hw_ctx = new_hctxs kfree(hctxs) movslq %ebx,%rax mov (%rdx,%rax,8),%rdi ->uaf This problem was found by code review, and I comfirmed that the concurrent scenario do exist(specifically 'q->queue_hw_ctx' can be changed during blk_mq_run_hw_queues()), however, the uaf problem hasn't been repoduced yet without hacking the kernel. Sicne the queue is freezed in __blk_mq_update_nr_hw_queues(), fix the problem by protecting 'queue_hw_ctx' through rcu where it can be accessed without grabbing 'q_usage_counter'. [1] https://lore.kernel.org/all/20220225072053.2472431-1-yukuai3@huawei.com/ Signed-off-by: Yu Kuai Signed-off-by: Fengnan Chang Signed-off-by: Jens Axboe --- block/blk-mq.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) (limited to 'block') diff --git a/block/blk-mq.c b/block/blk-mq.c index 1ef81110eb8a..4e96bb246247 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -4535,7 +4535,12 @@ static void __blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set, if (hctxs) memcpy(new_hctxs, hctxs, q->nr_hw_queues * sizeof(*hctxs)); - q->queue_hw_ctx = new_hctxs; + rcu_assign_pointer(q->queue_hw_ctx, new_hctxs); + /* + * Make sure reading the old queue_hw_ctx from other + * context concurrently won't trigger uaf. + */ + synchronize_rcu_expedited(); kfree(hctxs); hctxs = new_hctxs; } -- cgit v1.2.3