nbd: Don't kill server on client that doesn't request TLS

Upstream NBD documents (as of commit 4feebc95) that servers MAY
choose to operate in a conditional mode, where it is up to the
client whether to use TLS.  For qemu's case, we want to always be
in FORCEDTLS mode, because of the risk of man-in-the-middle
attacks, and since we never export more than one device; likewise,
the qemu client will ALWAYS send NBD_OPT_STARTTLS as its first
option.  But now that SELECTIVETLS servers exist, it is feasible
to encounter a (non-qemu) client that is programmed to talk to
such a server, and does not do NBD_OPT_STARTTLS first, but rather
wants to probe if it can use a non-encrypted export.

The NBD protocol documents that we should let such a client
continue trying, on the grounds that maybe the client will get the
hint to send NBD_OPT_STARTTLS, rather than immediately dropping
the connection.

Note that NBD_OPT_EXPORT_NAME is a special case: since it is the
only option request that can't have an error return, we have to
(continue to) drop the connection on that one; rather, what we are
fixing here is that all other replies prior to TLS initiation tell
the client NBD_REP_ERR_TLS_REQD, but keep the connection alive.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-id: 1460671343-18485-1-git-send-email-eblake@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

nbd/server.c

@@ -449,11 +449,19 @@ static int nbd_negotiate_options(NBDClient *client)
                 client->ioc = QIO_CHANNEL(tioc);
                 break;
 
+            case NBD_OPT_EXPORT_NAME:
+                /* No way to return an error to client, so drop connection */
+                TRACE("Option 0x%x not permitted before TLS", clientflags);
+                return -EINVAL;
+
             default:
                 TRACE("Option 0x%x not permitted before TLS", clientflags);
+                if (nbd_negotiate_drop_sync(client->ioc, length) != length) {
+                    return -EIO;
+                }
                 nbd_negotiate_send_rep(client->ioc, NBD_REP_ERR_TLS_REQD,
                                        clientflags);
-                return -EINVAL;
+                break;
             }
         } else if (fixedNewstyle) {
             switch (clientflags) {
@@ -471,6 +479,9 @@ static int nbd_negotiate_options(NBDClient *client)
                 return nbd_negotiate_handle_export_name(client, length);
 
             case NBD_OPT_STARTTLS:
+                if (nbd_negotiate_drop_sync(client->ioc, length) != length) {
+                    return -EIO;
+                }
                 if (client->tlscreds) {
                     TRACE("TLS already enabled");
                     nbd_negotiate_send_rep(client->ioc, NBD_REP_ERR_INVALID,
@@ -480,7 +491,7 @@ static int nbd_negotiate_options(NBDClient *client)
                     nbd_negotiate_send_rep(client->ioc, NBD_REP_ERR_POLICY,
                                            clientflags);
                 }
-                return -EINVAL;
+                break;
             default:
                 TRACE("Unsupported option 0x%x", clientflags);
                 if (nbd_negotiate_drop_sync(client->ioc, length) != length) {