nbd: fix use-after-free of rq/bio in the xmit path

For writes, we can get a completion in while we're still iterating
the request and bio chain. If that happens, we're reading freed
memory and we can crash.

Break out after the last segment and avoid having the iterator
read freed memory.
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/nbd.c

@@ -272,6 +272,7 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd)
 	int result, flags;
 	struct nbd_request request;
 	unsigned long size = blk_rq_bytes(req);
+	struct bio *bio;
 	u32 type;
 
 	if (req->cmd_type == REQ_TYPE_DRV_PRIV)
@@ -305,16 +306,20 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd)
 		return -EIO;
 	}
 
-	if (type == NBD_CMD_WRITE) {
-		struct req_iterator iter;
-		struct bio_vec bvec;
-		/*
-		 * we are really probing at internals to determine
-		 * whether to set MSG_MORE or not...
-		 */
-		rq_for_each_segment(bvec, req, iter) {
+	if (type != NBD_CMD_WRITE)
+		return 0;
+
 	flags = 0;
-			if (!rq_iter_last(bvec, iter))
+	bio = req->bio;
+	while (bio) {
+		struct bio *next = bio->bi_next;
+		struct bvec_iter iter;
+		struct bio_vec bvec;
+
+		bio_for_each_segment(bvec, bio, iter) {
+			bool is_last = !next && bio_iter_last(bvec, iter);
+
+			if (is_last)
 				flags = MSG_MORE;
 			dev_dbg(nbd_to_dev(nbd), "request %p: sending %d bytes data\n",
 				cmd, bvec.bv_len);
@@ -325,7 +330,16 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd)
 					result);
 				return -EIO;
 			}
+			/*
+			 * The completion might already have come in,
+			 * so break for the last one instead of letting
+			 * the iterator do it. This prevents use-after-free
+			 * of the bio.
+			 */
+			if (is_last)
+				break;
 		}
+		bio = next;
 	}
 	return 0;
 }