Re: [Qemu-devel] PATCH 6/8: x509 client certificate verification

["Daniel P. Berrange" <berrange@redhat.com>]

This patch adds support for requesting and validating client certificates.
In effect the client certificates are being used as the authentication
mechansim, making VNC passwords unccessary, though using a combination of
both is also possible.

The verification of client certificates is enabled using the 'x509verify'
flag to VNC arg eg  '-vnc :1,tls,x509verify'

diff -r f8cd8bd742ee vnc.c
--- a/vnc.c	Tue Jul 31 11:45:22 2007 -0400
+++ b/vnc.c	Tue Jul 31 11:45:30 2007 -0400
@@ -37,7 +37,7 @@
 #include <gnutls/x509.h>
 #endif /* CONFIG_VNC_TLS */
 
-// #define _VNC_DEBUG 1
+ #define _VNC_DEBUG 1
 
 #if _VNC_DEBUG
 #define VNC_DEBUG(fmt, ...) do { fprintf(stderr, fmt, ## __VA_ARGS__); } while (0)
@@ -141,6 +141,7 @@ struct VncState
     int auth;
 #if CONFIG_VNC_TLS
     int subauth;
+    int x509verify;
 #endif
     char challenge[VNC_AUTH_CHALLENGE_SIZE];
 
@@ -1418,6 +1419,85 @@ static gnutls_certificate_credentials_t 
     return x509_cred;
 }
 
+static int vnc_validate_certificate(struct VncState *vs)
+{
+    int ret;
+    unsigned int status;
+    const gnutls_datum_t *certs;
+    unsigned int nCerts, i;
+    time_t now;
+
+    VNC_DEBUG("Validating client certificate\n");
+    if ((ret = gnutls_certificate_verify_peers2 (vs->tls_session, &status)) < 0) {
+	VNC_DEBUG("Verify failed %s\n", gnutls_strerror(ret));
+	return -1;
+    }
+
+    if ((now = time(NULL)) == ((time_t)-1)) {
+	return -1;
+    }
+
+    if (status != 0) {
+	if (status & GNUTLS_CERT_INVALID)
+	    VNC_DEBUG("The certificate is not trusted.\n");
+
+	if (status & GNUTLS_CERT_SIGNER_NOT_FOUND)
+	    VNC_DEBUG("The certificate hasn't got a known issuer.\n");
+
+	if (status & GNUTLS_CERT_REVOKED)
+	    VNC_DEBUG("The certificate has been revoked.\n");
+
+	if (status & GNUTLS_CERT_INSECURE_ALGORITHM)
+	    VNC_DEBUG("The certificate uses an insecure algorithm\n");
+
+	return -1;
+    } else {
+	VNC_DEBUG("Certificate is valid!\n");
+    }
+
+    /* Only support x509 for now */
+    if (gnutls_certificate_type_get(vs->tls_session) != GNUTLS_CRT_X509)
+	return -1;
+
+    if (!(certs = gnutls_certificate_get_peers(vs->tls_session, &nCerts)))
+	return -1;
+
+    for (i = 0 ; i < nCerts ; i++) {
+	gnutls_x509_crt_t cert;
+	VNC_DEBUG ("Checking certificate chain %d\n", i);
+	if (gnutls_x509_crt_init (&cert) < 0)
+	    return -1;
+
+	if (gnutls_x509_crt_import(cert, &certs[i], GNUTLS_X509_FMT_DER) < 0) {
+	    gnutls_x509_crt_deinit (cert);
+	    return -1;
+	}
+
+	if (gnutls_x509_crt_get_expiration_time (cert) < now) {
+	    VNC_DEBUG("The certificate has expired\n");
+	    gnutls_x509_crt_deinit (cert);
+	    return -1;
+	}
+
+	if (gnutls_x509_crt_get_activation_time (cert) > now) {
+	    VNC_DEBUG("The certificate is not yet activated\n");
+	    gnutls_x509_crt_deinit (cert);
+	    return -1;
+	}
+
+	if (gnutls_x509_crt_get_activation_time (cert) > now) {
+	    VNC_DEBUG("The certificate is not yet activated\n");
+	    gnutls_x509_crt_deinit (cert);
+	    return -1;
+	}
+
+	gnutls_x509_crt_deinit (cert);
+    }
+
+    return 0;
+}
+
+
 static int start_auth_vencrypt_subauth(VncState *vs)
 {
     switch (vs->subauth) {
@@ -1464,6 +1544,16 @@ static int vnc_continue_handshake(struct
        VNC_DEBUG("Handshake failed %s\n", gnutls_strerror(ret));
        vnc_client_error(vs);
        return -1;
+    }
+
+    if (vs->x509verify) {
+	if (vnc_validate_certificate(vs) < 0) {
+	    VNC_DEBUG("Client verification failed\n");
+	    vnc_client_error(vs);
+	    return -1;
+	} else {
+	    VNC_DEBUG("Client verification passed\n");
+	}
     }
 
     VNC_DEBUG("Handshake done, switching to TLS data mode\n");
@@ -1547,6 +1637,11 @@ static int vnc_start_tls(struct VncState
 		vnc_client_error(vs);
 		return -1;
 	    }
+	    if (vs->x509verify) {
+		VNC_DEBUG("Requesting a client certificate\n");
+		gnutls_certificate_server_set_request (vs->tls_session, GNUTLS_CERT_REQUEST);
+	    }
+
 	} else {
 	    gnutls_anon_server_credentials anon_cred = vnc_tls_initialize_anon_cred();
 	    if (!anon_cred) {
@@ -1808,6 +1903,7 @@ void vnc_display_close(DisplayState *ds)
     vs->auth = VNC_AUTH_INVALID;
 #if CONFIG_VNC_TLS
     vs->subauth = VNC_AUTH_INVALID;
+    vs->x509verify = 0;
 #endif
     if (vs->lsock != -1) {
 	close(vs->lsock);
@@ -1842,10 +1938,14 @@ int vnc_display_open(DisplayState *ds, c
     options = display;
     while ((options = strchr(options, ','))) {
 	options++;
-	if (strncmp(options, "tls", 3) == 0)
+	if (strncmp(options, "tls", 3) == 0) {
 	    tls = 1; /* Require TLS */
-	else if (strncmp(options, "x509", 4) == 0)
+	} else if (strncmp(options, "x509verify", 10) == 0) {
+	    x509 = 1;
+	    vs->x509verify = 1; /* Require x509 certificates & verify client */
+	} else if (strncmp(options, "x509", 4) == 0) {
 	    x509 = 1; /* Require x509 certificates */
+	}
     }
 #endif
 

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