[gtk-vnc] Protect against divide by zero in MSLogon auth

[Daniel P. Berrange <dberrange src gnome org>]

commit f0fc819ae879f449c8d5dcb09ee623f4df15ed82
Author: Daniel P. Berrangé <berrange redhat com>
Date:   Fri Aug 17 11:18:52 2018 +0100

    Protect against divide by zero in MSLogon auth
    
    Signed-off-by: Daniel P. Berrangé <berrange redhat com>

 src/vncconnection.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
---
diff --git a/src/vncconnection.c b/src/vncconnection.c
index a2fa1da..5c03d70 100644
--- a/src/vncconnection.c
+++ b/src/vncconnection.c
@@ -3768,6 +3768,8 @@ static gboolean vnc_connection_perform_auth_mslogon(VncConnection *conn)
     gcry_mpi_t genmpi, modmpi, respmpi, pubmpi, keympi;
     guchar username[256], password[64];
     guint passwordLen, usernameLen;
+    gboolean allzeroes = TRUE;
+    int i;
 
     VNC_DEBUG("Do Challenge");
     priv->want_cred_password = TRUE;
@@ -3783,6 +3785,18 @@ static gboolean vnc_connection_perform_auth_mslogon(VncConnection *conn)
     if (vnc_connection_has_error(conn))
         return FALSE;
 
+    /* If 'mod' is bogus all-zeros when we get a divide by zero
+     * so sanity check that */
+    for (i = 0; i < 8; i++) {
+        if (mod[i])
+            allzeroes = FALSE;
+    }
+    if (allzeroes) {
+        vnc_connection_set_error(conn, "%s",
+                                 "Bad DH modulus value");
+        return FALSE;
+    }
+
     genmpi = vnc_bytes_to_mpi(gen,sizeof(gen));
     modmpi = vnc_bytes_to_mpi(mod,sizeof(mod));
     respmpi = vnc_bytes_to_mpi(resp,sizeof(resp));