Cleanup handling of initial auth token

This changes that auth_token_initial is set when the token is
initially generated instead when pushing the token. Even I do not
know anymore why I did it in this way in the first place. Also use
multi->auth_token_initial as source for the sesssion ID since it should
now always be available. Also set auth_token_initial directly to
up->password once we verified that we have gotten a valid token from
a client. This cleans ups the logic in generating the environment and
makes the code flow clearer.

Since the change makes auth_token_initial always available we need to add
a check to only send a PUSH reply to update the token on renegotiations.
The old code relied on multi->auth_token not being set in this case.

This commit also removes the workaround for old OpenVPN clients. These
were only available as commercial OpenVPN Connect client and not in use
anymore.

Furthermore, introduce a check if the session ID has changed during a
session.  Even though this is still a valid authentication changing to
a different auth token mid session is highly irregular and should never
occur naturally.

Patch V2: rebase.
Patch V3: fix formatting, clarifying commit message, remove initial
          token workaround for old v3.
Patch v4: move sending the auth-token for renegotiations to a sane place
          and trigger it when the TLS session reaches its fully authenticated
          state.
Patch v5: Move also setting auth_token_inital from up->password to a more
          logical place, general cleanups, add session id mismatch check
Patch v6: Rework some comments and general cleanup of small things

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20210719133132.128783-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22645.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

8 files changed, 141 insertions, 69 deletions

diff --git a/doc/man-sections/server-options.rst b/doc/man-sections/server-options.rst
index 7154733..f1d2ec3 100644
--- a/doc/man-sections/server-options.rst
+++ b/doc/man-sections/server-options.rst
@@ -35,7 +35,7 @@ fast hardware. SSL/TLS authentication must be used in this mode.
   token is reached or after not being renewed for more than 2 \*
   ``reneg-sec`` seconds. Clients will be sent renewed tokens on every TLS
   renogiation to keep the client's token updated. This is done to
-  invalidate a token if a client is disconnected for a sufficently long
+  invalidate a token if a client is disconnected for a sufficiently long
   time, while at the same time permitting much longer token lifetimes for
   active clients.
 
@@ -46,7 +46,7 @@ fast hardware. SSL/TLS authentication must be used in this mode.
   When the :code:`external-auth` keyword is present the normal
   authentication method will always be called even if auth-token succeeds.
   Normally other authentications method are skipped if auth-token
-  verification suceeds or fails.
+  verification succeeds or fails.
 
   This option postpones this decision to the external authentication
   methods and checks the validity of the account and do other checks.
diff --git a/src/openvpn/auth_token.c b/src/openvpn/auth_token.c
index 0ea6d18..c6c37ea 100644
--- a/src/openvpn/auth_token.c
+++ b/src/openvpn/auth_token.c
@@ -21,6 +21,8 @@
 const char *auth_token_pem_name = "OpenVPN auth-token server key";
 
 #define AUTH_TOKEN_SESSION_ID_LEN 12
+#define AUTH_TOKEN_SESSION_ID_BASE64_LEN (AUTH_TOKEN_SESSION_ID_LEN * 8 / 6)
+
 #if AUTH_TOKEN_SESSION_ID_LEN % 3
 #error AUTH_TOKEN_SESSION_ID_LEN needs to be multiple a 3
 #endif
@@ -109,11 +111,11 @@ add_session_token_env(struct tls_session *session, struct tls_multi *multi,
         /*
          * No session before, generate a new session token for the new session
          */
-        if (!multi->auth_token)
+        if (!multi->auth_token_initial)
         {
             generate_auth_token(up, multi);
         }
-        session_id_source = multi->auth_token;
+        session_id_source = multi->auth_token_initial;
     }
     /*
      * In the auth-token the auth token is already base64 encoded
@@ -184,7 +186,7 @@ generate_auth_token(const struct user_pass *up, struct tls_multi *multi)
 
     uint8_t sessid[AUTH_TOKEN_SESSION_ID_LEN];
 
-    if (multi->auth_token)
+    if (multi->auth_token_initial)
     {
         /* Just enough space to fit 8 bytes+ 1 extra to decode a non padded
          * base64 string (multiple of 3 bytes). 9 bytes => 12 bytes base64
@@ -192,13 +194,16 @@ generate_auth_token(const struct user_pass *up, struct tls_multi *multi)
          */
         char old_tstamp_decode[9];
 
-        /*
-         * reuse the same session id and timestamp and null terminate it at
-         * for base64 decode it only decodes the session id part of it
-         */
-        char *old_sessid = multi->auth_token + strlen(SESSION_ID_PREFIX);
+        /* Make a copy of the string to not modify multi->auth_token_initial */
+        char *initial_token_copy = string_alloc(multi->auth_token_initial, &gc);
+
+        char *old_sessid = initial_token_copy + strlen(SESSION_ID_PREFIX);
         char *old_tsamp_initial = old_sessid + AUTH_TOKEN_SESSION_ID_LEN*8/6;
 
+        /*
+         * We null terminate the old token just after the session ID to let
+         * our base64 decode function only decode the session ID
+         */
         old_tsamp_initial[12] = '\0';
         ASSERT(openvpn_base64_decode(old_tsamp_initial, old_tstamp_decode, 9) == 9);
 
@@ -211,11 +216,7 @@ generate_auth_token(const struct user_pass *up, struct tls_multi *multi)
         initial_timestamp = *tstamp_ptr;
 
         old_tsamp_initial[0] = '\0';
-        ASSERT(openvpn_base64_decode(old_sessid, sessid, AUTH_TOKEN_SESSION_ID_LEN)==AUTH_TOKEN_SESSION_ID_LEN);
-
-
-        /* free the auth-token, we will replace it with a new one */
-        free(multi->auth_token);
+        ASSERT(openvpn_base64_decode(old_sessid, sessid, AUTH_TOKEN_SESSION_ID_LEN) == AUTH_TOKEN_SESSION_ID_LEN);
     }
     else if (!rand_bytes(sessid, AUTH_TOKEN_SESSION_ID_LEN))
     {
@@ -272,11 +273,22 @@ generate_auth_token(const struct user_pass *up, struct tls_multi *multi)
 
     free(b64output);
 
+    /* free the auth-token if defined, we will replace it with a new one */
+    free(multi->auth_token);
     multi->auth_token = strdup((char *)BPTR(&session_token));
 
     dmsg(D_SHOW_KEYS, "Generated token for client: %s (%s)",
          multi->auth_token, up->username);
 
+    if (!multi->auth_token_initial)
+    {
+        /*
+         * Save the initial auth token to continue using the same session ID
+         * and timestamp in updates
+         */
+        multi->auth_token_initial = strdup(multi->auth_token);
+    }
+
     gc_free(&gc);
 }
 
@@ -343,23 +355,17 @@ verify_auth_token(struct user_pass *up, struct tls_multi *multi,
     }
     else
     {
-        msg(M_WARN, "--auth-token-gen: HMAC on token from client failed (%s)",
+        msg(M_WARN, "--auth-gen-token: HMAC on token from client failed (%s)",
             up->username);
         return 0;
     }
 
     /* Accept session tokens that not expired are in the acceptable range
      * for renogiations */
-    bool in_renog_time = now >= timestamp
-                         && now < timestamp + 2 * session->opt->renegotiate_seconds;
-
-    /* We could still have a client that does not update
-     * its auth-token, so also allow the initial auth-token */
-    bool initialtoken = multi->auth_token_initial
-                        && memcmp_constant_time(up->password, multi->auth_token_initial,
-                                                strlen(multi->auth_token_initial)) == 0;
+    bool in_renegotiation_time = now >= timestamp
+                                 && now < timestamp + 2 * session->opt->renegotiate_seconds;
 
-    if (!in_renog_time && !initialtoken)
+    if (!in_renegotiation_time)
     {
         ret |= AUTH_TOKEN_EXPIRED;
     }
@@ -383,7 +389,20 @@ verify_auth_token(struct user_pass *up, struct tls_multi *multi,
     {
         /* Tell client that the session token is expired */
         auth_set_client_reason(multi, "SESSION: token expired");
-        msg(M_INFO, "--auth-token-gen: auth-token from client expired");
+        msg(M_INFO, "--auth-gen-token: auth-token from client expired");
+    }
+
+    /* Check that we do have the same session ID in the token as in our stored
+     * auth-token to ensure that it did not change.
+     * This also compares the prefix and session part of the
+     * tokens, which should be identical if the session ID stayed the same */
+    if (multi->auth_token_initial
+        && memcmp_constant_time(multi->auth_token_initial, up->password,
+                                strlen(SESSION_ID_PREFIX) + AUTH_TOKEN_SESSION_ID_BASE64_LEN))
+    {
+        msg(M_WARN, "--auth-gen-token: session id in token changed (Rejecting "
+                    "token.");
+        ret = 0;
     }
     return ret;
 }
@@ -408,3 +427,27 @@ wipe_auth_token(struct tls_multi *multi)
         multi->auth_token_initial = NULL;
     }
 }
+
+void
+resend_auth_token_renegotiation(struct tls_multi *multi, struct tls_session *session)
+{
+    /*
+     * Auth token already sent to client, update auth-token on client.
+     * The initial auth-token is sent as part of the push message, for this
+     * update we need to schedule an extra push message.
+     *
+     * Otherwise the auth-token get pushed out as part of the "normal"
+     * push-reply
+     */
+    bool is_renegotiation = session->key[KS_PRIMARY].key_id != 0;
+
+    if (multi->auth_token_initial && is_renegotiation)
+    {
+        /*
+         * We do not explicitly reschedule the sending of the
+         * control message here. This might delay this reply
+         * a few seconds but this message is not time critical
+         */
+        send_push_reply_auth_token(multi);
+    }
+}
diff --git a/src/openvpn/auth_token.h b/src/openvpn/auth_token.h
index 73a00dd..de93a94 100644
--- a/src/openvpn/auth_token.h
+++ b/src/openvpn/auth_token.h
@@ -117,7 +117,7 @@ void wipe_auth_token(struct tls_multi *multi);
 /**
  * Return if the password string has the format of a password.
  *
- * This fuction will always read as many bytes as SESSION_ID_PREFIX is longer
+ * This function will always read as many bytes as SESSION_ID_PREFIX is longer
  * the caller needs ensure that password memory is at least that long (true for
  * calling with struct user_pass)
  * @param password
@@ -129,4 +129,13 @@ is_auth_token(const char *password)
     return (memcmp_constant_time(SESSION_ID_PREFIX, password,
                                  strlen(SESSION_ID_PREFIX)) == 0);
 }
+/**
+ * Checks if a client should be sent a new auth token to update its
+ * current auth-token
+ * @param multi     Pointer the multi object of the TLS session
+ * @param session   Pointer to the TLS session itself
+ */
+void
+resend_auth_token_renegotiation(struct tls_multi *multi, struct tls_session *session);
+
 #endif /* AUTH_TOKEN_H */
diff --git a/src/openvpn/push.c b/src/openvpn/push.c
index f4957f1..53cb7ca 100644
--- a/src/openvpn/push.c
+++ b/src/openvpn/push.c
@@ -527,14 +527,6 @@ prepare_auth_token_push_reply(struct tls_multi *tls_multi, struct gc_arena *gc,
         push_option_fmt(gc, push_list, M_USAGE,
                         "auth-token %s",
                         tls_multi->auth_token);
-        if (!tls_multi->auth_token_initial)
-        {
-            /*
-             * Save the initial auth token for clients that ignore
-             * the updates to the token
-             */
-            tls_multi->auth_token_initial = strdup(tls_multi->auth_token);
-        }
     }
 }
 
diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index 615ed69..e9b8d1b 100644
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -3115,13 +3115,18 @@ tls_multi_process(struct tls_multi *multi,
 
             if (ks->state == S_ACTIVE && ks->authenticated == KS_AUTH_TRUE)
             {
-                /* This will move ks->state from S_ACTIVE to S_GENERATED_KEYS */
+                /* Session is now fully authenticated.
+                 * tls_session_generate_data_channel_keys will move ks->state
+                 * from S_ACTIVE to S_GENERATED_KEYS */
                 if (!tls_session_generate_data_channel_keys(session))
                 {
                     msg(D_TLS_ERRORS, "TLS Error: generate_key_expansion failed");
                     ks->authenticated = KS_AUTH_FALSE;
                     ks->state = S_ERROR;
                 }
+
+                /* Update auth token on the client if needed */
+                resend_auth_token_renegotiation(multi, session);
             }
         }
     }
diff --git a/src/openvpn/ssl_common.h b/src/openvpn/ssl_common.h
index 72eb55b..64c1d53 100644
--- a/src/openvpn/ssl_common.h
+++ b/src/openvpn/ssl_common.h
@@ -95,7 +95,10 @@
                                  *   completed while still within the
                                  *   handshake window.  Deferred auth and
                                  *   client connect can still be pending. */
-#define S_GENERATED_KEYS  7     /**< The data channel keys have been generated */
+#define S_GENERATED_KEYS  7     /**< The data channel keys have been generated
+                                  *  The TLS session is fully authenticated
+                                  *  when reaching this state. */
+
 /* Note that earlier versions also had a S_OP_NORMAL state that was
  * virtually identical with S_ACTIVE and the code still assumes everything
  * >= S_ACTIVE to be fully operational */
@@ -596,8 +599,8 @@ struct tls_multi
                           *   user/pass authentications in this session.
                           */
     char *auth_token_initial;
-    /**< The first auth-token we sent to a client, for clients that do
-     * not update their auth-token (older OpenVPN3 core versions)
+    /**< The first auth-token we sent to a client. We use this to remember
+     * the session ID and initial timestamp when generating new auth-token.
      */
 #define  AUTH_TOKEN_HMAC_OK              (1<<0)
     /**< Auth-token sent from client has valid hmac */
diff --git a/src/openvpn/ssl_verify.c b/src/openvpn/ssl_verify.c
index bbb1878..913c956 100644
--- a/src/openvpn/ssl_verify.c
+++ b/src/openvpn/ssl_verify.c
@@ -1512,6 +1512,15 @@ verify_user_pass(struct user_pass *up, struct tls_multi *multi,
     if (session->opt->auth_token_generate && is_auth_token(up->password))
     {
         ks->auth_token_state_flags = verify_auth_token(up, multi, session);
+
+        /* If this is the first time we see an auth-token in this multi session,
+         * save it as initial auth token. This ensures using the
+         * same session ID and initial timestamp in new tokens */
+        if (!multi->auth_token_initial)
+        {
+            multi->auth_token_initial = strdup(up->password);
+        }
+
         if (session->opt->auth_token_call_auth)
         {
             /*
@@ -1631,27 +1640,7 @@ verify_user_pass(struct user_pass *up, struct tls_multi *multi,
              */
             generate_auth_token(up, multi);
         }
-        /*
-         * Auth token already sent to client, update auth-token on client.
-         * The initial auth-token is sent as part of the push message, for this
-         * update we need to schedule an extra push message.
-         *
-         * Otherwise the auth-token get pushed out as part of the "normal"
-         * push-reply
-         */
-        if (multi->auth_token_initial)
-        {
-            /*
-             * We do not explicitly schedule the sending of the
-             * control message here but control message are only
-             * postponed when the control channel  is not yet fully
-             * established and furthermore since this is called in
-             * the middle of authentication, there are other messages
-             * (new data channel keys) that are sent anyway and will
-             * trigger schedueling
-             */
-            send_push_reply_auth_token(multi);
-        }
+
         msg(D_HANDSHAKE, "TLS: Username/Password authentication %s for username '%s' %s",
             (ks->authenticated == KS_AUTH_DEFERRED) ? "deferred" : "succeeded",
             up->username,
diff --git a/tests/unit_tests/openvpn/test_auth_token.c b/tests/unit_tests/openvpn/test_auth_token.c
index 4030052..6bfad6e 100644
--- a/tests/unit_tests/openvpn/test_auth_token.c
+++ b/tests/unit_tests/openvpn/test_auth_token.c
@@ -174,7 +174,10 @@ auth_token_test_timeout(void **state)
 
     now = 100000;
     generate_auth_token(&ctx->up, &ctx->multi);
+
     strcpy(ctx->up.password, ctx->multi.auth_token);
+    free(ctx->multi.auth_token_initial);
+    ctx->multi.auth_token_initial = NULL;
 
     /* No time has passed */
     assert_int_equal(verify_auth_token(&ctx->up, &ctx->multi, ctx->session),
@@ -195,11 +198,6 @@ auth_token_test_timeout(void **state)
     assert_int_equal(verify_auth_token(&ctx->up, &ctx->multi, ctx->session),
                      AUTH_TOKEN_HMAC_OK|AUTH_TOKEN_EXPIRED);
 
-    /* Check if the mode for a client that never updates its token works */
-    ctx->multi.auth_token_initial = strdup(ctx->up.password);
-    assert_int_equal(verify_auth_token(&ctx->up, &ctx->multi, ctx->session),
-                     AUTH_TOKEN_HMAC_OK);
-
     /* But not when we reached our timeout */
     now = 100000 + ctx->session->opt->auth_token_lifetime + 1;
     assert_int_equal(verify_auth_token(&ctx->up, &ctx->multi, ctx->session),
@@ -244,10 +242,10 @@ auth_token_test_known_keys(void **state)
 
     now = 0;
     /* Preload the session id so the same session id is used here */
-    ctx->multi.auth_token = strdup(now0key0);
+    ctx->multi.auth_token_initial = strdup(now0key0);
 
     /* Zero the hmac part to ensure we have a newly generated token */
-    zerohmac(ctx->multi.auth_token);
+    zerohmac(ctx->multi.auth_token_initial);
 
     generate_auth_token(&ctx->up, &ctx->multi);
 
@@ -268,6 +266,38 @@ setenv_str(struct env_set *es, const char *name, const char *value)
     }
 }
 
+void
+auth_token_test_session_mismatch(void **state)
+{
+    struct test_context *ctx = (struct test_context *) *state;
+
+    /* Generate first auth token and check it is correct */
+    generate_auth_token(&ctx->up, &ctx->multi);
+    strcpy(ctx->up.password, ctx->multi.auth_token);
+    assert_int_equal(verify_auth_token(&ctx->up, &ctx->multi, ctx->session),
+                     AUTH_TOKEN_HMAC_OK);
+
+    char *token_sessiona = strdup(ctx->multi.auth_token);
+
+    /* Generate second token */
+    wipe_auth_token(&ctx->multi);
+
+    generate_auth_token(&ctx->up, &ctx->multi);
+    strcpy(ctx->up.password, ctx->multi.auth_token);
+    assert_int_equal(verify_auth_token(&ctx->up, &ctx->multi, ctx->session),
+                     AUTH_TOKEN_HMAC_OK);
+
+    assert_int_not_equal(0, memcmp(ctx->multi.auth_token_initial + strlen(SESSION_ID_PREFIX),
+                                   token_sessiona + strlen(SESSION_ID_PREFIX),
+                                   AUTH_TOKEN_SESSION_ID_BASE64_LEN));
+
+    /* The first token is valid but should trigger the invalid response since
+     * the session id is not the same */
+    strcpy(ctx->up.password, token_sessiona);
+    assert_int_equal(verify_auth_token(&ctx->up, &ctx->multi, ctx->session), 0);
+    free(token_sessiona);
+}
+
 static void
 auth_token_test_empty_user(void **state)
 {
@@ -341,13 +371,13 @@ auth_token_test_random_keys(void **state)
 
     now = 0x5c331e9c;
     /* Preload the session id so the same session id is used here */
-    ctx->multi.auth_token = strdup(random_token);
+    ctx->multi.auth_token_initial = strdup(random_token);
 
     free_key_ctx(&ctx->multi.opt.auth_token_key);
     auth_token_init_secret(&ctx->multi.opt.auth_token_key, random_key, true);
 
     /* Zero the hmac part to ensure we have a newly generated token */
-    zerohmac(ctx->multi.auth_token);
+    zerohmac(ctx->multi.auth_token_initial);
 
     generate_auth_token(&ctx->up, &ctx->multi);
 
@@ -385,6 +415,7 @@ main(void)
         cmocka_unit_test_setup_teardown(auth_token_test_random_keys, setup, teardown),
         cmocka_unit_test_setup_teardown(auth_token_test_key_load, setup, teardown),
         cmocka_unit_test_setup_teardown(auth_token_test_timeout, setup, teardown),
+        cmocka_unit_test_setup_teardown(auth_token_test_session_mismatch, setup, teardown)
     };
 
 #if defined(ENABLE_CRYPTO_OPENSSL)