Implement using --peer-fingerprint without CA certificates

This is implements --peer-fingerprint command to support OpenVPN
authentication without involving a PKI.

The current implementation in OpenVPN for peer fingerprint has been already
extensively rewritten from the original submission from Jason [1]. The
commit preserved the original author since it was based on Jason code/idea.

This commit is based on two previous commits that prepare the infrastructure
to use a simple to use --peer-fingerprint directive instead of using
a --tls-verify script like the v1 of the patch proposed.  The two commits
preparing this are:

 - Extend verify-hash to allow multiple hashes
 - Implement peer-fingerprint to check fingerprint of peer certificate

These preceding patches make this actual patch quite short. There are some
lines in this patch that bear some similarity to the ones like

    if (!preverify_ok && !session->opt->verify_hash_no_ca)

vs

    if (!preverify_ok && !session->opt->ca_file_none)

But these similarities are one line fragments and dictated by the
surrounding style and program flow, so even a complete black box
implementation will likely end up with the same lines.

[1] https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16781.html

Change-Id: Ie74c3d606c5429455c293c367462244566a936e3
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230524132424.3098475-2-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26723.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit c3746da7f04acf872f251d3673551963380c4d77)

6 files changed, 33 insertions, 14 deletions

diff --git a/src/openvpn/init.c b/src/openvpn/init.c
index c023b33..d358ad0 100644
--- a/src/openvpn/init.c
+++ b/src/openvpn/init.c
@@ -3347,6 +3347,7 @@ do_init_crypto_tls(struct context *c, const unsigned int flags)
     to.verify_hash = options->verify_hash;
     to.verify_hash_algo = options->verify_hash_algo;
     to.verify_hash_depth = options->verify_hash_depth;
+    to.verify_hash_no_ca = options->verify_hash_no_ca;
 #ifdef ENABLE_X509ALTUSERNAME
     memcpy(to.x509_username_field, options->x509_username_field, sizeof(to.x509_username_field));
 #else
diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 6d8ae57..efddc58 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -2989,21 +2989,11 @@ options_postprocess_verify_ce(const struct options *options,
         else
         {
 #ifdef ENABLE_CRYPTO_MBEDTLS
-            if (!(options->ca_file))
-            {
-                msg(M_USAGE, "You must define CA file (--ca)");
-            }
-
             if (options->ca_path)
             {
                 msg(M_USAGE, "Parameter --capath cannot be used with the mbed TLS version version of OpenVPN.");
             }
-#else  /* ifdef ENABLE_CRYPTO_MBEDTLS */
-            if ((!(options->ca_file)) && (!(options->ca_path)))
-            {
-                msg(M_USAGE, "You must define CA file (--ca) or CA path (--capath)");
-            }
-#endif
+#endif  /* ifdef ENABLE_CRYPTO_MBEDTLS */
             if (pull)
             {
 
@@ -3735,6 +3725,13 @@ options_postprocess_mutate(struct options *o, struct env_set *es)
         options_postprocess_http_proxy_override(o);
     }
 #endif
+    if (!o->ca_file && !o->ca_path && o->verify_hash
+        && o->verify_hash_depth == 0)
+    {
+        msg(M_INFO, "Using certificate fingerprint to verify peer (no CA "
+            "option set). ");
+        o->verify_hash_no_ca = true;
+    }
 
     if (o->config && streq(o->config, "stdin") && o->remap_sigusr1 == SIGHUP)
     {
@@ -4030,8 +4027,11 @@ options_postprocess_filechecks(struct options *options)
     errs |= check_file_access_inline(options->dh_file_inline, CHKACC_FILE,
                                      options->dh_file, R_OK, "--dh");
 
-    errs |= check_file_access_inline(options->ca_file_inline, CHKACC_FILE,
-                                     options->ca_file, R_OK, "--ca");
+    if (!options->verify_hash_no_ca)
+    {
+        errs |= check_file_access_inline(options->ca_file_inline, CHKACC_FILE,
+                                         options->ca_file, R_OK, "--ca");
+    }
 
     errs |= check_file_access_chroot(options->chroot_dir, CHKACC_FILE,
                                      options->ca_path, R_OK, "--capath");
diff --git a/src/openvpn/options.h b/src/openvpn/options.h
index 95f1158..f5890b9 100644
--- a/src/openvpn/options.h
+++ b/src/openvpn/options.h
@@ -604,6 +604,7 @@ struct options
     struct verify_hash_list *verify_hash;
     hash_algo_type verify_hash_algo;
     int verify_hash_depth;
+    bool verify_hash_no_ca;
     unsigned int ssl_flags; /* set to SSLF_x flags from ssl.h */
 
 #ifdef ENABLE_PKCS11
diff --git a/src/openvpn/ssl_common.h b/src/openvpn/ssl_common.h
index c0b3caa..27b0294 100644
--- a/src/openvpn/ssl_common.h
+++ b/src/openvpn/ssl_common.h
@@ -345,6 +345,7 @@ struct tls_options
     const char *remote_cert_eku;
     struct verify_hash_list *verify_hash;
     int verify_hash_depth;
+    bool verify_hash_no_ca;
     hash_algo_type verify_hash_algo;
 #ifdef ENABLE_X509ALTUSERNAME
     char *x509_username_field[MAX_PARMS];
diff --git a/src/openvpn/ssl_verify_mbedtls.c b/src/openvpn/ssl_verify_mbedtls.c
index c9ef7a1..e3437f7 100644
--- a/src/openvpn/ssl_verify_mbedtls.c
+++ b/src/openvpn/ssl_verify_mbedtls.c
@@ -62,6 +62,22 @@ verify_callback(void *session_obj, mbedtls_x509_crt *cert, int cert_depth,
     struct buffer cert_fingerprint = x509_get_sha256_fingerprint(cert, &gc);
     cert_hash_remember(session, cert_depth, &cert_fingerprint);
 
+    if (session->opt->verify_hash_no_ca)
+    {
+        /*
+         * If we decide to verify the peer certificate based on the fingerprint
+         * we ignore wrong dates and the certificate not being trusted.
+         * Any other problem with the certificate (wrong key, bad cert,...)
+         * will still trigger an error.
+         * Clearing these flags relies on verify_cert will later rejecting a
+         * certificate that has no matching fingerprint.
+         */
+        uint32_t flags_ignore = MBEDTLS_X509_BADCERT_NOT_TRUSTED
+                                | MBEDTLS_X509_BADCERT_EXPIRED
+                                | MBEDTLS_X509_BADCERT_FUTURE;
+        *flags = *flags & ~flags_ignore;
+    }
+
     /* did peer present cert which was signed by our root cert? */
     if (*flags != 0)
     {
diff --git a/src/openvpn/ssl_verify_openssl.c b/src/openvpn/ssl_verify_openssl.c
index ac36f09..e24ce4e 100644
--- a/src/openvpn/ssl_verify_openssl.c
+++ b/src/openvpn/ssl_verify_openssl.c
@@ -67,7 +67,7 @@ verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
     cert_hash_remember(session, X509_STORE_CTX_get_error_depth(ctx), &cert_hash);
 
     /* did peer present cert which was signed by our root cert? */
-    if (!preverify_ok)
+    if (!preverify_ok && !session->opt->verify_hash_no_ca)
     {
         /* get the X509 name */
         char *subject = x509_get_subject(current_cert, &gc);