[Spice-devel] Graceful failure when VNC client connects to SPICE server ?

[Frediano Ziglio]

> 
> On Wed, Jan 25, 2017 at 12:23:35PM -0500, Frediano Ziglio wrote:
> > > 
> > > When used with QEMU at least, both SPICE and VNC live in the same port
> > > range (5900+). It is thus not entirely uncommon for a user to mistakenly
> > > connect to a SPICE server with a VNC client, or vica-verca.
> > > 
> > > When connecting to VNC server with a SPICE client, you quickly get an
> > > error. This is because the VNC server sends its short greeting and then
> > > sees the RedLinkMess from the SPICE client, realizes its garbage and
> > > drops the connection.
> > > 
> > > When connecting to a SPICE server with a VNC client though, you get an
> > > indefinite hang. The VNC client is waiting for the VNC greeting, which
> > > the SPICE server will never send. The SPICE server is waiting for the
> > > RedLinkMess which the VNC client will never send.
> > > 
> > > Since VNC is a server sends first protocol and SPICE is a client sends
> > > first protocol, it would seem this problem is impossible to fix, but
> > > I think it is possible to tweak things so it can be more gracefully
> > > handled.
> > > 
> > > In VNC the protocol starts with the follow data sent:
> > > 
> > >  Server: "RFB 003.008\n"
> > >  Client: "RFB 003.008\n"
> > > 
> > > What I'm thinking is that we could easily change the VNC client so that
> > > it will send some bytes first. eg
> > > 
> > >  Client: "RFB "
> > >  Server: "RFB 003.008\n"
> > >  Client: "003.008\n"
> > > 
> > > From the VNC server POV, it'll still be receiving the same 12 bytes from
> > > the client - it just happens that 4 of them might arrive a tiny bit
> > > earlier than the other 8 of them. IOW nothing should break in the VNC
> > > server from this change.
> > > 
> > > I tried this, but we still get a hang in the SPICE server. The problem
> > > is that the SPICE server code is waiting for the full RedLinkMess
> > > message to be received before checking its content.
> > > 
> > > Can we change the SPICE server so that it reads the 'uint32 magic' first,
> > > validates that, and then reads the remainder of RedLinkMess ?
> > > 
> > > This would solve the problem, as the SPICE server would see 'RFB ' as the
> > > magic, realize it was garbage and so close the connection. The VNC client
> > > which is waiting for the VNC greeting will thus terminate rather than
> > > hanging
> > > forever.
> > > 
> > > Regards,
> > > Daniel
> > 
> > Perhaps another solution would be to add a timeout to spice server so
> > after, say, 2/3 seconds it would close the connection.
> > 
> > This would work even without changing the VNC client.
> 
> I'm not a fan of timeouts because this could negatively impact on
> genuine spice clients which hit a latency spike on the network
> for whatever reason. The incremental read of RedLinkMess by constrast
> is guaranteed to not have any chance of negative impact on spice clients.
> 
> Regards,
> Daniel

So you would like something like that:


diff --git a/server/reds.c b/server/reds.c
index 8db70ee..50098df 100644
--- a/server/reds.c
+++ b/server/reds.c
@@ -2264,12 +2264,6 @@ static void reds_handle_read_header_done(void *opaque)
     header->minor_version = GUINT32_FROM_LE(header->minor_version);
     header->size = GUINT32_FROM_LE(header->size);
 
-    if (header->magic != SPICE_MAGIC) {
-        reds_send_link_error(link, SPICE_LINK_ERR_INVALID_MAGIC);
-        reds_link_free(link);
-        return;
-    }
-
     if (header->major_version != SPICE_VERSION_MAJOR) {
         if (header->major_version > 0) {
             reds_send_link_error(link, SPICE_LINK_ERR_VERSION_MISMATCH);
@@ -2296,13 +2290,31 @@ static void reds_handle_read_header_done(void *opaque)
                            link);
 }
 
+static void reds_handle_read_magic_done(void *opaque)
+{
+    RedLinkInfo *link = (RedLinkInfo *)opaque;
+    const SpiceLinkHeader *header = &link->link_header;
+
+    if (header->magic != SPICE_MAGIC) {
+        reds_send_link_error(link, SPICE_LINK_ERR_INVALID_MAGIC);
+        reds_link_free(link);
+        return;
+    }
+
+    reds_stream_async_read(link->stream,
+                           ((uint8_t *)&link->link_header) + sizeof(header->magic),
+                           sizeof(SpiceLinkHeader) - sizeof(header->magic),
+                           reds_handle_read_header_done,
+                           link);
+}
+
 static void reds_handle_new_link(RedLinkInfo *link)
 {
     reds_stream_set_async_error_handler(link->stream, reds_handle_link_error);
     reds_stream_async_read(link->stream,
                            (uint8_t *)&link->link_header,
-                           sizeof(SpiceLinkHeader),
-                           reds_handle_read_header_done,
+                           sizeof(link->link_header.magic),
+                           reds_handle_read_magic_done,
                            link);
 }
 

?

I don't think the extra code execute is much of a problem.

Frediano