Re: [PATCH RFC] blk-mq: fix potential uaf for 'queue_hw_ctx'

[Ming Lei]

On Wed, Feb 23, 2022 at 07:26:01PM +0800, Yu Kuai wrote:
> 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
> 

Not only uaf on queue_hw_ctx, but also other similar issue on other
structures, and I think the correct and easy fix is to quiesce request
queue during updating nr_hw_queues, something like the following patch:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index a05ce7725031..d8e7c3cce0dd 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -4467,8 +4467,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
 	if (set->nr_maps == 1 && nr_hw_queues == set->nr_hw_queues)
 		return;
 
-	list_for_each_entry(q, &set->tag_list, tag_set_list)
+	list_for_each_entry(q, &set->tag_list, tag_set_list) {
 		blk_mq_freeze_queue(q);
+		blk_mq_quiesce_queue(q);
+	}
 	/*
 	 * Switch IO scheduler to 'none', cleaning up the data associated
 	 * with the previous scheduler. We will switch back once we are done
@@ -4518,8 +4520,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
 	list_for_each_entry(q, &set->tag_list, tag_set_list)
 		blk_mq_elv_switch_back(&head, q);
 
-	list_for_each_entry(q, &set->tag_list, tag_set_list)
+	list_for_each_entry(q, &set->tag_list, tag_set_list) {
+		blk_mq_unquiesce_queue(q);
 		blk_mq_unfreeze_queue(q);
+	}
 }
 
 void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues)



Thanks,
Ming