mmc: fix use-after-free of struct request

We call mmc_req_is_special() after having processed a request, but
it could be freed after that. Check that ahead of time, and use
the cached value.

Reported-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Fixes: c2df40dfb8c0 ("drivers: use req op accessor")

Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/mmc/card/block.c

@@ -2151,6 +2151,7 @@ static int mmc_blk_issue_rq(struct mmc_queue *mq, struct request *req)
 	struct mmc_card *card = md->queue.card;
 	struct mmc_host *host = card->host;
 	unsigned long flags;
+	bool req_is_special = mmc_req_is_special(req);
 
 	if (req && !mq->mqrq_prev->req)
 		/* claim host only for the first request */
@@ -2191,8 +2192,7 @@ static int mmc_blk_issue_rq(struct mmc_queue *mq, struct request *req)
 	}
 
 out:
-	if ((!req && !(mq->flags & MMC_QUEUE_NEW_REQUEST)) ||
-	    mmc_req_is_special(req))
+	if ((!req && !(mq->flags & MMC_QUEUE_NEW_REQUEST)) || req_is_special)
 		/*
 		 * Release host when there are no more requests
 		 * and after special request(discard, flush) is done.

drivers/mmc/card/queue.c

@@ -65,6 +65,8 @@ static int mmc_queue_thread(void *d)
 		spin_unlock_irq(q->queue_lock);
 
 		if (req || mq->mqrq_prev->req) {
+			bool req_is_special = mmc_req_is_special(req);
+
 			set_current_state(TASK_RUNNING);
 			mq->issue_fn(mq, req);
 			cond_resched();
@@ -80,7 +82,7 @@ static int mmc_queue_thread(void *d)
 			 * has been finished. Do not assign it to previous
 			 * request.
 			 */
-			if (mmc_req_is_special(req))
+			if (req_is_special)
 				mq->mqrq_cur->req = NULL;
 
 			mq->mqrq_prev->brq.mrq.data = NULL;