Refactor key_state_export_keying_material functions

This refactors the common code between mbed SSL and OpenSSL into
export_user_keying_material and also prepares the backend functions
to export more than one key.

Also fix checking the return value of SSL_export_keying_material
only 1 is a success, -1 is also an error.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>

Patch V2: Cache secrets for mbed TLS instead generating all ekms
          in the call back function

Patch V3: comment is no longer a lie. (fixed doxygen)

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <20200814145153.12895-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20739.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

src/openvpn/ssl.c

@@ -2412,6 +2412,40 @@ error:
     return false;
 }
 
+static void
+export_user_keying_material(struct key_state_ssl *ssl,
+                            struct tls_session *session)
+{
+    if (session->opt->ekm_size > 0)
+    {
+        unsigned int size = session->opt->ekm_size;
+        struct gc_arena gc = gc_new();
+
+        unsigned char *ekm;
+        if ((ekm = key_state_export_keying_material(session,
+                                                    session->opt->ekm_label,
+                                                    session->opt->ekm_label_size,
+                                                    session->opt->ekm_size,
+                                                    &gc)))
+        {
+            unsigned int len = (size * 2) + 2;
+
+            const char *key = format_hex_ex(ekm, size, len, 0, NULL, &gc);
+            setenv_str(session->opt->es, "exported_keying_material", key);
+
+            dmsg(D_TLS_DEBUG_MED, "%s: exported keying material: %s",
+                 __func__, key);
+            secure_memzero(ekm, size);
+        }
+        else
+        {
+            msg(M_WARN, "WARNING: Export keying material failed!");
+            setenv_del(session->opt->es, "exported_keying_material");
+        }
+        gc_free(&gc);
+    }
+}
+
 /**
  * Handle reading key data, peer-info, username/password, OCC
  * from the TLS control channel (cleartext).
@@ -2541,7 +2575,7 @@ key_method_2_read(struct buffer *buf, struct tls_multi *multi, struct tls_sessio
     if ((ks->authenticated > KS_AUTH_FALSE)
         && plugin_defined(session->opt->plugins, OPENVPN_PLUGIN_TLS_FINAL))
     {
-        key_state_export_keying_material(&ks->ks_ssl, session);
+        export_user_keying_material(&ks->ks_ssl, session);
 
         if (plugin_call(session->opt->plugins, OPENVPN_PLUGIN_TLS_FINAL, NULL, NULL, session->opt->es) != OPENVPN_PLUGIN_FUNC_SUCCESS)
         {

src/openvpn/ssl_backend.h

@@ -394,13 +394,21 @@ void backend_tls_ctx_reload_crl(struct tls_root_ctx *ssl_ctx,
  * derived from existing TLS channel. This exported keying material can then be
  * used for a variety of purposes.
  *
- * @param ks_ssl       The SSL channel's state info
  * @param session      The session associated with the given key_state
+ * @param label        The label to use when exporting the key
+ * @param label_size   The size of the label to use when exporting the key
+ * @param ekm_size     THe size of the exported/returned key material
+ * @param gc           gc_arena that might be used to allocate the string
+ *                     returned
+ * @returns            The exported key material, the caller may zero the
+ *                     string but should not free it
  */
 
-void
+unsigned char*
-key_state_export_keying_material(struct key_state_ssl *ks_ssl,
+key_state_export_keying_material(struct tls_session *session,
-                                 struct tls_session *session) __attribute__((nonnull));
+                                 const char* label, size_t label_size,
+                                 size_t ekm_size,
+                                 struct gc_arena *gc) __attribute__((nonnull));
 
 /**************************************************************************/
 /** @addtogroup control_tls

src/openvpn/ssl_mbedtls.c

@@ -206,52 +206,55 @@ mbedtls_ssl_export_keys_cb(void *p_expkey, const unsigned char *ms,
 {
     struct tls_session *session = p_expkey;
     struct key_state_ssl *ks_ssl = &session->key[KS_PRIMARY].ks_ssl;
-    unsigned char client_server_random[64];
+    struct tls_key_cache *cache = &ks_ssl->tls_key_cache;
 
-    ks_ssl->exported_key_material = gc_malloc(session->opt->ekm_size,
+    static_assert(sizeof(ks_ssl->ctx->session->master)
-                                              true, NULL);
+                    == sizeof(cache->master_secret), "master size mismatch");
 
-    memcpy(client_server_random, client_random, 32);
+    memcpy(cache->client_server_random, client_random, 32);
-    memcpy(client_server_random + 32, server_random, 32);
+    memcpy(cache->client_server_random + 32, server_random, 32);
+    memcpy(cache->master_secret, ms, sizeof(cache->master_secret));
+    cache->tls_prf_type = tls_prf_type;
 
-    const size_t ms_len = sizeof(ks_ssl->ctx->session->master);
+    return true;
-    int ret = mbedtls_ssl_tls_prf(tls_prf_type, ms, ms_len,
+}
-                                  session->opt->ekm_label, client_server_random,
-                                  sizeof(client_server_random), ks_ssl->exported_key_material,
-                                  session->opt->ekm_size);
 
-    if (!mbed_ok(ret))
+unsigned char *
+key_state_export_keying_material(struct tls_session *session,
+                                 const char* label, size_t label_size,
+                                 size_t ekm_size,
+                                 struct gc_arena *gc)
+{
+    ASSERT(strlen(label) == label_size);
+
+    struct tls_key_cache *cache = &session->key[KS_PRIMARY].ks_ssl.tls_key_cache;
+
+    /* If the type is NONE, we either have no cached secrets or
+     * there is no PRF, in both cases we cannot generate key material */
+    if (cache->tls_prf_type == MBEDTLS_SSL_TLS_PRF_NONE)
     {
-        secure_memzero(ks_ssl->exported_key_material, session->opt->ekm_size);
+        return NULL;
     }
 
-    secure_memzero(client_server_random, sizeof(client_server_random));
+    unsigned char *ekm = (unsigned char *) gc_malloc(ekm_size, true, gc);
+    int ret = mbedtls_ssl_tls_prf(cache->tls_prf_type, cache->master_secret,
+                                  sizeof(cache->master_secret),
+                                  label, cache->client_server_random,
+                                  sizeof(cache->client_server_random),
+                                  ekm, ekm_size);
 
-    return ret;
+    if (mbed_ok(ret))
+    {
+        return ekm;
+    }
+    else
+    {
+        secure_memzero(ekm, session->opt->ekm_size);
+        return  NULL;
+    }
 }
 #endif /* HAVE_EXPORT_KEYING_MATERIAL */
 
-void
-key_state_export_keying_material(struct key_state_ssl *ssl,
-                                 struct tls_session *session)
-{
-    if (ssl->exported_key_material)
-    {
-        unsigned int size = session->opt->ekm_size;
-        struct gc_arena gc = gc_new();
-        unsigned int len = (size * 2) + 2;
-
-        const char *key = format_hex_ex(ssl->exported_key_material,
-                                        size, len, 0, NULL, &gc);
-        setenv_str(session->opt->es, "exported_keying_material", key);
-
-        dmsg(D_TLS_DEBUG_MED, "%s: exported keying material: %s",
-             __func__, key);
-        gc_free(&gc);
-    }
-}
-
-
 bool
 tls_ctx_set_options(struct tls_root_ctx *ctx, unsigned int ssl_flags)
 {
@@ -1178,7 +1181,7 @@ key_state_ssl_free(struct key_state_ssl *ks_ssl)
 {
     if (ks_ssl)
     {
-        free(ks_ssl->exported_key_material);
+        CLEAR(ks_ssl->tls_key_cache);
 
         if (ks_ssl->ctx)
         {

src/openvpn/ssl_mbedtls.h

@@ -82,6 +82,15 @@ struct external_context {
     void *sign_ctx;
 };
 
+/** struct to cache TLS secrets for keying material exporter (RFC 5705).
+ * The constants (64 and 48) are inherent to TLS version and
+ * the whole keying material export will likely change when they change */
+struct tls_key_cache {
+    unsigned char client_server_random[64];
+    mbedtls_tls_prf_types tls_prf_type;
+    unsigned char master_secret[48];
+};
+
 /**
  * Structure that wraps the TLS context. Contents differ depending on the
  * SSL library used.
@@ -114,8 +123,7 @@ struct key_state_ssl {
     mbedtls_ssl_context *ctx;           /**< mbedTLS connection context */
     bio_ctx *bio_ctx;
 
-    /** Keying material exporter cache (RFC 5705). */
+    struct tls_key_cache tls_key_cache;
-    uint8_t *exported_key_material;
 
 };
 

src/openvpn/ssl_openssl.c

@@ -158,35 +158,26 @@ tls_ctx_initialised(struct tls_root_ctx *ctx)
     return NULL != ctx->ctx;
 }
 
-void
+unsigned char*
-key_state_export_keying_material(struct key_state_ssl *ssl,
+key_state_export_keying_material(struct tls_session *session,
-                                 struct tls_session *session)
+                                 const char* label, size_t label_size,
+                                 size_t ekm_size,
+                                 struct gc_arena *gc)
+
 {
-    if (session->opt->ekm_size > 0)
+    unsigned char *ekm = (unsigned char *) gc_malloc(ekm_size, true, gc);
+
+    SSL* ssl = session->key[KS_PRIMARY].ks_ssl.ssl;
+
+    if (SSL_export_keying_material(ssl, ekm, ekm_size, label,
+                                   label_size, NULL, 0, 0) == 1)
     {
-        unsigned int size = session->opt->ekm_size;
+        return ekm;
-        struct gc_arena gc = gc_new();
+    }
-        unsigned char *ekm = (unsigned char *) gc_malloc(size, true, &gc);
+    else
-
+    {
-        if (SSL_export_keying_material(ssl->ssl, ekm, size,
+        secure_memzero(ekm, ekm_size);
-                                       session->opt->ekm_label,
+        return NULL;
-                                       session->opt->ekm_label_size,
-                                       NULL, 0, 0))
-        {
-            unsigned int len = (size * 2) + 2;
-
-            const char *key = format_hex_ex(ekm, size, len, 0, NULL, &gc);
-            setenv_str(session->opt->es, "exported_keying_material", key);
-
-            dmsg(D_TLS_DEBUG_MED, "%s: exported keying material: %s",
-                 __func__, key);
-        }
-        else
-        {
-            msg(M_WARN, "WARNING: Export keying material failed!");
-            setenv_del(session->opt->es, "exported_keying_material");
-        }
-        gc_free(&gc);
     }
 }