bfq-iosched: ensure to clear bic/bfqq pointers when preparing request

Even if we don't have an IO context attached to a request, we still
need to clear the priv[0..1] pointers, as they could be pointing
to previously used bic/bfqq structures. If we don't do so, we'll
either corrupt memory on dispatching a request, or cause an
imbalance in counters.

Inspired by a fix from Kees.
Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Reported-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
Fixes: aee69d78 ("block, bfq: introduce the BFQ-v0 I/O scheduler as an extra scheduler")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -4934,8 +4934,16 @@ static void bfq_prepare_request(struct request *rq, struct bio *bio)
 	bool new_queue = false;
 	bool bfqq_already_existing = false, split = false;
 
-	if (!rq->elv.icq)
+	/*
+	 * Even if we don't have an icq attached, we should still clear
+	 * the scheduler pointers, as they might point to previously
+	 * allocated bic/bfqq structs.
+	 */
+	if (!rq->elv.icq) {
+		rq->elv.priv[0] = rq->elv.priv[1] = NULL;
 		return;
+	}
+
 	bic = icq_to_bic(rq->elv.icq);
 
 	spin_lock_irq(&bfqd->lock);