[PATCH 7/8] blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter

[Bart Van Assche]

On 4/25/21 1:57 AM, Ming Lei wrote:
> However, still one request UAF not covered: refcount_inc_not_zero() may
> read one freed request, and it will be handled in next patch.

This means that patch "blk-mq: clear stale request in tags->rq[] before
freeing one request pool" should come before this patch.

> @@ -276,12 +277,15 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
>  		rq = tags->static_rqs[bitnr];
>  	else
>  		rq = tags->rqs[bitnr];
> -	if (!rq)
> +	if (!rq || !refcount_inc_not_zero(&rq->ref))
>  		return true;
>  	if ((iter_data->flags & BT_TAG_ITER_STARTED) &&
>  	    !blk_mq_request_started(rq))
> -		return true;
> -	return iter_data->fn(rq, iter_data->data, reserved);
> +		ret = true;
> +	else
> +		ret = iter_data->fn(rq, iter_data->data, reserved);
> +	blk_mq_put_rq_ref(rq);
> +	return ret;
>  }

Even if patches 7/8 and 8/8 would be reordered, the above code
introduces a new use-after-free, a use-after-free that is much worse
than the UAF in kernel v5.11. The following sequence can be triggered by
the above code:
* bt_tags_iter() reads tags->rqs[bitnr] and stores the request pointer
in the 'rq' variable.
* Request 'rq' completes, tags->rqs[bitnr] is cleared and the memory
that backs that request is freed.
* The memory that backs 'rq' is used for another purpose and the request
reference count becomes nonzero.
* bt_tags_iter() increments the request reference count and thereby
corrupts memory.

Bart.