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

Bart Van Assche
2021-04-25T18:55:31Z

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.