GSSAPI key exchange support

This patch has been rejected upstream: "None of the OpenSSH developers are
in favour of adding this, and this situation has not changed for several
years.  This is not a slight on Simon's patch, which is of fine quality, but
just that a) we don't trust GSSAPI implementations that much and b) we don't
like adding new KEX since they are pre-auth attack surface.  This one is
particularly scary, since it requires hooks out to typically root-owned
system resources."

However, quite a lot of people rely on this in Debian, and it's better to
have it merged into the main openssh package rather than having separate
-krb5 packages (as we used to have).  It seems to have a generally good
security history.

Bug: https://bugzilla.mindrot.org/show_bug.cgi?id=1242
Last-Updated: 2017-01-16

Patch-Name: gssapi.patch

1 files changed, 45 insertions, 3 deletions

diff --git a/auth2-gss.c b/auth2-gss.c
index 1ca835773..3b5036dfd 100644
--- a/auth2-gss.c
+++ b/auth2-gss.c
@@ -1,7 +1,7 @@
 /* $OpenBSD: auth2-gss.c,v 1.22 2015/01/19 20:07:45 markus Exp $ */
 
 /*
- * Copyright (c) 2001-2003 Simon Wilkinson. All rights reserved.
+ * Copyright (c) 2001-2007 Simon Wilkinson. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -53,6 +53,40 @@ static int input_gssapi_mic(int type, u_int32_t plen, void *ctxt);
 static int input_gssapi_exchange_complete(int type, u_int32_t plen, void *ctxt);
 static int input_gssapi_errtok(int, u_int32_t, void *);
 
+/* 
+ * The 'gssapi_keyex' userauth mechanism.
+ */
+static int
+userauth_gsskeyex(Authctxt *authctxt)
+{
+        int authenticated = 0;
+        Buffer b;
+        gss_buffer_desc mic, gssbuf;
+        u_int len;
+
+        mic.value = packet_get_string(&len);
+        mic.length = len;
+
+        packet_check_eom();
+
+        ssh_gssapi_buildmic(&b, authctxt->user, authctxt->service,
+            "gssapi-keyex");
+
+        gssbuf.value = buffer_ptr(&b);
+        gssbuf.length = buffer_len(&b);
+
+        /* gss_kex_context is NULL with privsep, so we can't check it here */
+        if (!GSS_ERROR(PRIVSEP(ssh_gssapi_checkmic(gss_kex_context, 
+            &gssbuf, &mic))))
+                authenticated = PRIVSEP(ssh_gssapi_userok(authctxt->user,
+                    authctxt->pw));
+        
+        buffer_free(&b);
+        free(mic.value);
+
+        return (authenticated);
+}
+
 /*
  * We only support those mechanisms that we know about (ie ones that we know
  * how to check local user kuserok and the like)
@@ -238,7 +272,8 @@ input_gssapi_exchange_complete(int type, u_int32_t plen, void *ctxt)
 
         packet_check_eom();
 
-        authenticated = PRIVSEP(ssh_gssapi_userok(authctxt->user));
+        authenticated = PRIVSEP(ssh_gssapi_userok(authctxt->user,
+            authctxt->pw));
 
         authctxt->postponed = 0;
         dispatch_set(SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
@@ -274,7 +309,8 @@ input_gssapi_mic(int type, u_int32_t plen, void *ctxt)
         gssbuf.length = buffer_len(&b);
 
         if (!GSS_ERROR(PRIVSEP(ssh_gssapi_checkmic(gssctxt, &gssbuf, &mic))))
-                authenticated = PRIVSEP(ssh_gssapi_userok(authctxt->user));
+                authenticated = 
+                    PRIVSEP(ssh_gssapi_userok(authctxt->user, authctxt->pw));
         else
                 logit("GSSAPI MIC check failed");
 
@@ -290,6 +326,12 @@ input_gssapi_mic(int type, u_int32_t plen, void *ctxt)
         return 0;
 }
 
+Authmethod method_gsskeyex = {
+        "gssapi-keyex",
+        userauth_gsskeyex,
+        &options.gss_authentication
+};
+
 Authmethod method_gssapi = {
         "gssapi-with-mic",
         userauth_gssapi,