Re: [PATCH v6 6/6] nbd: fix uaf in nbd_handle_reply()
From: Christoph Hellwig <hch@infradead.org>
Date: 2021-09-15 08:22:10
Also in:
lkml
On Wed, Sep 15, 2021 at 04:15:37PM +0800, Yu Kuai wrote:
quoted hunk ↗ jump to hunk
+++ b/block/blk-core.c@@ -489,6 +489,7 @@ void blk_queue_exit(struct request_queue *q) { percpu_ref_put(&q->q_usage_counter); } +EXPORT_SYMBOL(blk_queue_exit);
These needs to be an EXPORT_SYMBOL_GPL. But more importantly it needs to be a separate properly documented patch, and this function needs to grow a kerneldoc comment as well.
+ /*
+ * Get q_usage_counter can prevent accessing freed request
+ * through blk_mq_tag_to_rq() in nbd_handle_reply(). If
+ * q_usage_counter is zero, then no request is inflight, which
+ * means something is wrong since we expect to find a request to
+ * complete here.
+ */
+ if (!percpu_ref_tryget(&q->q_usage_counter)) {
+ dev_err(disk_to_dev(nbd->disk), "%s: no io inflight\n",
+ __func__);
+ break;
+ }And this needs a properly documented wrapper as well.
+
cmd = nbd_handle_reply(nbd, args->index, &reply);
- if (IS_ERR(cmd))
+ if (IS_ERR(cmd)) {
+ blk_queue_exit(q);
break;
+ }
rq = blk_mq_rq_from_pdu(cmd);
if (likely(!blk_should_fake_timeout(rq->q)))
blk_mq_complete_request(rq);
+ blk_queue_exit(q);That being said I can't say I like how this exposed block layer internals. We don't really need a reference to the queue here anywhere, you just use it as a dumb debug check. If we really want to reuse (abuse?) q_usage_counter a helper to just grab a reference and immediately drop it might be a better fit.