Commit 89e1fb7c authored by Fengnan Chang's avatar Fengnan Chang Committed by Jens Axboe
Browse files

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: default avatarYu Kuai <yukuai3@huawei.com>
Signed-off-by: default avatarFengnan Chang <changfengnan@bytedance.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent d0c98769

block/blk-mq.c

+6 −1
Original line number Diff line number Diff line
@@ -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;

	}

include/linux/blk-mq.h

+12 −1
Original line number Diff line number Diff line
@@ -1015,9 +1015,20 @@ static inline void *blk_mq_rq_to_pdu(struct request *rq) 
	return rq + 1;

}



static inline struct blk_mq_hw_ctx *queue_hctx(struct request_queue *q, int id)

{

	struct blk_mq_hw_ctx *hctx;



	rcu_read_lock();

	hctx = rcu_dereference(q->queue_hw_ctx)[id];

	rcu_read_unlock();



	return hctx;

}



#define queue_for_each_hw_ctx(q, hctx, i)				\

	for ((i) = 0; (i) < (q)->nr_hw_queues &&			\

	     ({ hctx = (q)->queue_hw_ctx[i]; 1; }); (i)++)

	     ({ hctx = queue_hctx((q), i); 1; }); (i)++)



#define hctx_for_each_ctx(hctx, ctx, i)					\

	for ((i) = 0; (i) < (hctx)->nr_ctx &&				\

include/linux/blkdev.h

+1 −1
Original line number Diff line number Diff line
@@ -503,7 +503,7 @@ struct request_queue { 


	/* hw dispatch queues */

	unsigned int		nr_hw_queues;

	struct blk_mq_hw_ctx	**queue_hw_ctx;

	struct blk_mq_hw_ctx * __rcu *queue_hw_ctx;



	struct percpu_ref	q_usage_counter;

	struct lock_class_key	io_lock_cls_key;