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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 <yukuai3@huawei.com>
Signed-off-by: Fengnan Chang <changfengnan@bytedance.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Fengnan Chang 2025-11-28 16:53:14 +08:00 committed by Jens Axboe
parent d0c98769ee
commit 89e1fb7cef
3 changed files with 19 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
block/blk-mq.c
View File

@ -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;
}

13
include/linux/blk-mq.h
View File

@ -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 && \

2
include/linux/blkdev.h
View File

@ -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;