SSL: ssl_reject_handshake directive (ticket #195).

In some cases it might be needed to reject SSL handshake based on SNI
server name provided, for example, to make sure an invalid certificate
is not returned to clients trying to contact a name-based virtual server
without SSL configured.  Previously, a "ssl_ciphers aNULL;" was used for
this.  This workaround, however, is not compatible with TLSv1.3, in
particular, when using BoringSSL, where it is not possible to configure
TLSv1.3 ciphers at all.

With this change, the ssl_reject_handshake directive is introduced,
which instructs nginx to reject SSL handshakes with an "unrecognized_name"
alert in a particular server block.

For example, to reject handshake with names other than example.com,
one can use the following configuration:

    server {
        listen 443 ssl;
        ssl_reject_handshake on;
    }

    server {
        listen 443 ssl;
        server_name example.com;
        ssl_certificate example.com.crt;
        ssl_certificate_key example.com.key;
    }

The following configuration can be used to reject all SSL handshakes
without SNI server name provided:

    server {
        listen 443 ssl;
        ssl_reject_handshake on;
    }

    server {
        listen 443 ssl;
        server_name ~^;
        ssl_certificate example.crt;
        ssl_certificate_key example.key;
    }

Additionally, the ssl_reject_handshake directive makes configuring
certificates for the default server block optional.  If no certificates
are configured in the default server for a given listening socket,
certificates must be defined in all non-default server blocks with
the listening socket in question.

5 files changed, 104 insertions, 42 deletions

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index dfb816055..2eef87e5e 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -1793,6 +1793,13 @@ ngx_ssl_handshake(ngx_connection_t *c)
         return NGX_ERROR;
     }
 
+    if (c->ssl->handshake_rejected) {
+        ngx_connection_error(c, err, "handshake rejected");
+        ERR_clear_error();
+
+        return NGX_ERROR;
+    }
+
     c->read->error = 1;
 
     ngx_ssl_connection_error(c, sslerr, err, "SSL_do_handshake() failed");
@@ -3354,8 +3361,9 @@ ngx_ssl_session_id_context(ngx_ssl_t *ssl, ngx_str_t *sess_ctx,
         }
     }
 
-    if (SSL_CTX_get_ex_data(ssl->ctx, ngx_ssl_certificate_index) == NULL) {
-
+    if (SSL_CTX_get_ex_data(ssl->ctx, ngx_ssl_certificate_index) == NULL
+        && certificates != NULL)
+    {
         /*
          * If certificates are loaded dynamically, we use certificate
          * names as specified in the configuration (with variables).
diff --git a/src/event/ngx_event_openssl.h b/src/event/ngx_event_openssl.h
index 821bb13d1..329760d09 100644
--- a/src/event/ngx_event_openssl.h
+++ b/src/event/ngx_event_openssl.h
@@ -95,6 +95,7 @@ struct ngx_ssl_connection_s {
     u_char                      early_buf;
 
     unsigned                    handshaked:1;
+    unsigned                    handshake_rejected:1;
     unsigned                    renegotiation:1;
     unsigned                    buffer:1;
     unsigned                    no_wait_shutdown:1;
diff --git a/src/http/modules/ngx_http_ssl_module.c b/src/http/modules/ngx_http_ssl_module.c
index 2702f1e20..e062b03a1 100644
--- a/src/http/modules/ngx_http_ssl_module.c
+++ b/src/http/modules/ngx_http_ssl_module.c
@@ -294,6 +294,13 @@ static ngx_command_t  ngx_http_ssl_commands[] = {
       offsetof(ngx_http_ssl_srv_conf_t, conf_commands),
       &ngx_http_ssl_conf_command_post },
 
+    { ngx_string("ssl_reject_handshake"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_CONF_FLAG,
+      ngx_conf_set_flag_slot,
+      NGX_HTTP_SRV_CONF_OFFSET,
+      offsetof(ngx_http_ssl_srv_conf_t, reject_handshake),
+      NULL },
+
       ngx_null_command
 };
 
@@ -614,6 +621,7 @@ ngx_http_ssl_create_srv_conf(ngx_conf_t *cf)
     sscf->enable = NGX_CONF_UNSET;
     sscf->prefer_server_ciphers = NGX_CONF_UNSET;
     sscf->early_data = NGX_CONF_UNSET;
+    sscf->reject_handshake = NGX_CONF_UNSET;
     sscf->buffer_size = NGX_CONF_UNSET_SIZE;
     sscf->verify = NGX_CONF_UNSET_UINT;
     sscf->verify_depth = NGX_CONF_UNSET_UINT;
@@ -660,6 +668,7 @@ ngx_http_ssl_merge_srv_conf(ngx_conf_t *cf, void *parent, void *child)
                          prev->prefer_server_ciphers, 0);
 
     ngx_conf_merge_value(conf->early_data, prev->early_data, 0);
+    ngx_conf_merge_value(conf->reject_handshake, prev->reject_handshake, 0);
 
     ngx_conf_merge_bitmask_value(conf->protocols, prev->protocols,
                          (NGX_CONF_BITMASK_SET|NGX_SSL_TLSv1
@@ -707,38 +716,35 @@ ngx_http_ssl_merge_srv_conf(ngx_conf_t *cf, void *parent, void *child)
 
     if (conf->enable) {
 
-        if (conf->certificates == NULL) {
-            ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
-                          "no \"ssl_certificate\" is defined for "
-                          "the \"ssl\" directive in %s:%ui",
-                          conf->file, conf->line);
-            return NGX_CONF_ERROR;
-        }
+        if (conf->certificates) {
+            if (conf->certificate_keys == NULL) {
+                ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
+                              "no \"ssl_certificate_key\" is defined for "
+                              "the \"ssl\" directive in %s:%ui",
+                              conf->file, conf->line);
+                return NGX_CONF_ERROR;
+            }
 
-        if (conf->certificate_keys == NULL) {
-            ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
-                          "no \"ssl_certificate_key\" is defined for "
-                          "the \"ssl\" directive in %s:%ui",
-                          conf->file, conf->line);
-            return NGX_CONF_ERROR;
-        }
+            if (conf->certificate_keys->nelts < conf->certificates->nelts) {
+                ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
+                              "no \"ssl_certificate_key\" is defined "
+                              "for certificate \"%V\" and "
+                              "the \"ssl\" directive in %s:%ui",
+                              ((ngx_str_t *) conf->certificates->elts)
+                              + conf->certificates->nelts - 1,
+                              conf->file, conf->line);
+                return NGX_CONF_ERROR;
+            }
 
-        if (conf->certificate_keys->nelts < conf->certificates->nelts) {
+        } else if (!conf->reject_handshake) {
             ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
-                          "no \"ssl_certificate_key\" is defined "
-                          "for certificate \"%V\" and "
+                          "no \"ssl_certificate\" is defined for "
                           "the \"ssl\" directive in %s:%ui",
-                          ((ngx_str_t *) conf->certificates->elts)
-                          + conf->certificates->nelts - 1,
                           conf->file, conf->line);
             return NGX_CONF_ERROR;
         }
 
-    } else {
-
-        if (conf->certificates == NULL) {
-            return NGX_CONF_OK;
-        }
+    } else if (conf->certificates) {
 
         if (conf->certificate_keys == NULL
             || conf->certificate_keys->nelts < conf->certificates->nelts)
@@ -750,6 +756,9 @@ ngx_http_ssl_merge_srv_conf(ngx_conf_t *cf, void *parent, void *child)
                           + conf->certificates->nelts - 1);
             return NGX_CONF_ERROR;
         }
+
+    } else if (!conf->reject_handshake) {
+        return NGX_CONF_OK;
     }
 
     if (ngx_ssl_create(&conf->ssl, conf->protocols, conf) != NGX_OK) {
@@ -808,7 +817,7 @@ ngx_http_ssl_merge_srv_conf(ngx_conf_t *cf, void *parent, void *child)
         return NGX_CONF_ERROR;
 #endif
 
-    } else {
+    } else if (conf->certificates) {
 
         /* configure certificates */
 
@@ -947,6 +956,10 @@ ngx_http_ssl_compile_certificates(ngx_conf_t *cf,
     ngx_http_complex_value_t          *cv;
     ngx_http_compile_complex_value_t   ccv;
 
+    if (conf->certificates == NULL) {
+        return NGX_OK;
+    }
+
     cert = conf->certificates->elts;
     key = conf->certificate_keys->elts;
     nelts = conf->certificates->nelts;
@@ -1327,7 +1340,33 @@ ngx_http_ssl_init(ngx_conf_t *cf)
             cscf = addr[a].default_server;
             sscf = cscf->ctx->srv_conf[ngx_http_ssl_module.ctx_index];
 
-            if (sscf->certificates == NULL) {
+            if (sscf->certificates) {
+                continue;
+            }
+
+            if (!sscf->reject_handshake) {
+                ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
+                              "no \"ssl_certificate\" is defined for "
+                              "the \"listen ... ssl\" directive in %s:%ui",
+                              cscf->file_name, cscf->line);
+                return NGX_ERROR;
+            }
+
+            /*
+             * if no certificates are defined in the default server,
+             * check all non-default server blocks
+             */
+
+            cscfp = addr[a].servers.elts;
+            for (s = 0; s < addr[a].servers.nelts; s++) {
+
+                cscf = cscfp[s];
+                sscf = cscf->ctx->srv_conf[ngx_http_ssl_module.ctx_index];
+
+                if (sscf->certificates || sscf->reject_handshake) {
+                    continue;
+                }
+
                 ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
                               "no \"ssl_certificate\" is defined for "
                               "the \"listen ... ssl\" directive in %s:%ui",
diff --git a/src/http/modules/ngx_http_ssl_module.h b/src/http/modules/ngx_http_ssl_module.h
index 127570332..7ab0f7eae 100644
--- a/src/http/modules/ngx_http_ssl_module.h
+++ b/src/http/modules/ngx_http_ssl_module.h
@@ -21,6 +21,7 @@ typedef struct {
 
     ngx_flag_t                      prefer_server_ciphers;
     ngx_flag_t                      early_data;
+    ngx_flag_t                      reject_handshake;
 
     ngx_uint_t                      protocols;
 
diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
index 2a0528c68..204a9399d 100644
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -871,10 +871,14 @@ ngx_http_ssl_servername(ngx_ssl_conn_t *ssl_conn, int *ad, void *arg)
         return SSL_TLSEXT_ERR_ALERT_FATAL;
     }
 
+    hc = c->data;
+
     servername = SSL_get_servername(ssl_conn, TLSEXT_NAMETYPE_host_name);
 
     if (servername == NULL) {
-        return SSL_TLSEXT_ERR_OK;
+        ngx_log_debug0(NGX_LOG_DEBUG_HTTP, c->log, 0,
+                       "SSL server name: null");
+        goto done;
     }
 
     ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
@@ -883,7 +887,7 @@ ngx_http_ssl_servername(ngx_ssl_conn_t *ssl_conn, int *ad, void *arg)
     host.len = ngx_strlen(servername);
 
     if (host.len == 0) {
-        return SSL_TLSEXT_ERR_OK;
+        goto done;
     }
 
     host.data = (u_char *) servername;
@@ -891,32 +895,27 @@ ngx_http_ssl_servername(ngx_ssl_conn_t *ssl_conn, int *ad, void *arg)
     rc = ngx_http_validate_host(&host, c->pool, 1);
 
     if (rc == NGX_ERROR) {
-        *ad = SSL_AD_INTERNAL_ERROR;
-        return SSL_TLSEXT_ERR_ALERT_FATAL;
+        goto error;
     }
 
     if (rc == NGX_DECLINED) {
-        return SSL_TLSEXT_ERR_OK;
+        goto done;
     }
 
-    hc = c->data;
-
     rc = ngx_http_find_virtual_server(c, hc->addr_conf->virtual_names, &host,
                                       NULL, &cscf);
 
     if (rc == NGX_ERROR) {
-        *ad = SSL_AD_INTERNAL_ERROR;
-        return SSL_TLSEXT_ERR_ALERT_FATAL;
+        goto error;
     }
 
     if (rc == NGX_DECLINED) {
-        return SSL_TLSEXT_ERR_OK;
+        goto done;
     }
 
     hc->ssl_servername = ngx_palloc(c->pool, sizeof(ngx_str_t));
     if (hc->ssl_servername == NULL) {
-        *ad = SSL_AD_INTERNAL_ERROR;
-        return SSL_TLSEXT_ERR_ALERT_FATAL;
+        goto error;
     }
 
     *hc->ssl_servername = host;
@@ -933,8 +932,7 @@ ngx_http_ssl_servername(ngx_ssl_conn_t *ssl_conn, int *ad, void *arg)
 
     if (sscf->ssl.ctx) {
         if (SSL_set_SSL_CTX(ssl_conn, sscf->ssl.ctx) == NULL) {
-            *ad = SSL_AD_INTERNAL_ERROR;
-            return SSL_TLSEXT_ERR_ALERT_FATAL;
+            goto error;
         }
 
         /*
@@ -960,7 +958,22 @@ ngx_http_ssl_servername(ngx_ssl_conn_t *ssl_conn, int *ad, void *arg)
 #endif
     }
 
+done:
+
+    sscf = ngx_http_get_module_srv_conf(hc->conf_ctx, ngx_http_ssl_module);
+
+    if (sscf->reject_handshake) {
+        c->ssl->handshake_rejected = 1;
+        *ad = SSL_AD_UNRECOGNIZED_NAME;
+        return SSL_TLSEXT_ERR_ALERT_FATAL;
+    }
+
     return SSL_TLSEXT_ERR_OK;
+
+error:
+
+    *ad = SSL_AD_INTERNAL_ERROR;
+    return SSL_TLSEXT_ERR_ALERT_FATAL;
 }
 
 #endif