[Qemu-devel] [PATCH for-2.6] nbd: Don't kill server on client that doesn't request TLS

[Eric Blake <eblake@redhat.com>]

Upstream NBD is documenting 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 does not do NBD_OPT_STARTTLS first, but
rather wants to take advantage of the conditional modes it might
find elsewhere.

Since we require TLS, we are within our rights to drop connections
on any client that doesn't negotiate it right away, or which
attempts to negotiate it incorrectly, without violating the intent
of the NBD Protocol.  However, it's better to allow the client to
continue trying, on the grounds that maybe the client will get the
hint to send NBD_OPT_STARTTLS.

Signed-off-by: Eric Blake <eblake@redhat.com>
---

My earlier patch was arguably incomplete:
https://lists.gnu.org/archive/html/qemu-devel/2016-04/msg01265.html

But as it is already in a pull request, and as this one is
a bit more controversial, it's best to keep it as a separate patch.

 nbd/server.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 7843584..2b727f0 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -450,9 +450,12 @@ static int nbd_negotiate_options(NBDClient *client)

             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) {
@@ -470,6 +473,9 @@ 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,
@@ -479,7 +485,7 @@ 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) {
-- 
2.5.5