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

commit 429a787b upstream.

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>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/block/nbd.c

@@ -242,6 +242,7 @@ static int nbd_send_req(struct nbd_device *nbd, struct request *req)
 	int result, flags;
 	struct nbd_request request;
 	unsigned long size = blk_rq_bytes(req);
+	struct bio *bio;
 
 	memset(&request, 0, sizeof(request));
 	request.magic = htonl(NBD_REQUEST_MAGIC);
@@ -266,16 +267,20 @@ static int nbd_send_req(struct nbd_device *nbd, struct request *req)
 		goto error_out;
 	}
 
-	if (nbd_cmd(req) == NBD_CMD_WRITE) {
-		struct req_iterator iter;
+	if (nbd_cmd(req) != NBD_CMD_WRITE)
+		return 0;
+
+	flags = 0;
+	bio = req->bio;
+	while (bio) {
+		struct bio *next = bio->bi_next;
+		struct bvec_iter 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) {
-			flags = 0;
-			if (!rq_iter_last(bvec, iter))
+
+		bio_for_each_segment(bvec, bio, iter) {
+			bool is_last = !next && bio_iter_last(bvec, iter);
+
+			if (is_last)
 				flags = MSG_MORE;
 			dprintk(DBG_TX, "%s: request %p: sending %d bytes data\n",
 					nbd->disk->disk_name, req, bvec.bv_len);
@@ -286,7 +291,16 @@ static int nbd_send_req(struct nbd_device *nbd, struct request *req)
 					result);
 				goto error_out;
 			}
+			/*
+			 * 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;