nghttpx: Verify OCSP response

At least we should make sure that the OCSP response is targeted to the
expected certificate.  This is important because we pass the file path
to the external script, and if the file is replaced because of
renewal, and nghttpx has not reloaded its configuration, the
certificate nghttpx has loaded and the one included in the file
differ.  Verifying the OCSP response detects this, and avoids to send
wrong OCSP response.

src/shrpx_connection_handler.cc

@@ -620,8 +620,9 @@ void ConnectionHandler::handle_ocsp_complete() {
               << " finished successfully";
   }
 
+  if (tls::verify_ocsp_response(ssl_ctx, ocsp_.resp.data(),
+                                ocsp_.resp.size()) == 0) {
 #ifndef OPENSSL_IS_BORINGSSL
-  {
 #ifdef HAVE_ATOMIC_STD_SHARED_PTR
     std::atomic_store_explicit(
         &tls_ctx_data->ocsp_data,
@@ -632,10 +633,10 @@ void ConnectionHandler::handle_ocsp_complete() {
     tls_ctx_data->ocsp_data =
         std::make_shared<std::vector<uint8_t>>(std::move(ocsp_.resp));
 #endif // !HAVE_ATOMIC_STD_SHARED_PTR
-  }
 #else  // OPENSSL_IS_BORINGSSL
     SSL_CTX_set_ocsp_response(ssl_ctx, ocsp_.resp.data(), ocsp_.resp.size());
 #endif // OPENSSL_IS_BORINGSSL
+  }
 
   ++ocsp_.next;
   proceed_next_cert_ocsp();

src/shrpx_tls.cc

@@ -45,6 +45,7 @@
 #include <openssl/x509v3.h>
 #include <openssl/rand.h>
 #include <openssl/dh.h>
+#include <openssl/ocsp.h>
 
 #include <nghttp2/nghttp2.h>
 
@@ -1818,6 +1819,84 @@ int proto_version_from_string(const StringRef &v) {
   return -1;
 }
 
+int verify_ocsp_response(SSL_CTX *ssl_ctx, const uint8_t *ocsp_resp,
+                         size_t ocsp_resplen) {
+#if !defined(LIBRESSL_VERSION_NUMBER) && OPENSSL_VERSION_NUMBER >= 0x10002000L
+  int rv;
+
+  STACK_OF(X509) * chain_certs;
+  SSL_CTX_get0_chain_certs(ssl_ctx, &chain_certs);
+
+  auto resp = d2i_OCSP_RESPONSE(nullptr, &ocsp_resp, ocsp_resplen);
+  if (resp == nullptr) {
+    LOG(ERROR) << "d2i_OCSP_RESPONSE failed";
+    return -1;
+  }
+  auto resp_deleter = defer(OCSP_RESPONSE_free, resp);
+
+  ERR_clear_error();
+
+  auto bs = OCSP_response_get1_basic(resp);
+  if (bs == nullptr) {
+    LOG(ERROR) << "OCSP_response_get1_basic failed: "
+               << ERR_error_string(ERR_get_error(), nullptr);
+    return -1;
+  }
+  auto bs_deleter = defer(OCSP_BASICRESP_free, bs);
+
+  ERR_clear_error();
+
+  rv = OCSP_basic_verify(bs, chain_certs, nullptr, OCSP_TRUSTOTHER);
+
+  if (rv != 1) {
+    LOG(ERROR) << "OCSP_basic_verify failed: "
+               << ERR_error_string(ERR_get_error(), nullptr);
+    return -1;
+  }
+
+  auto sresp = OCSP_resp_get0(bs, 0);
+  if (sresp == nullptr) {
+    LOG(ERROR) << "OCSP response verification failed: no single response";
+    return -1;
+  }
+
+#if OPENSSL_1_1_API
+  auto certid = OCSP_SINGLERESP_get0_id(sresp);
+#else  // !OPENSSL_1_1_API
+  auto certid = sresp->certId;
+#endif // !OPENSSL_1_1_API
+  assert(certid != nullptr);
+
+  ASN1_INTEGER *serial;
+  rv = OCSP_id_get0_info(nullptr, nullptr, nullptr, &serial,
+                         const_cast<OCSP_CERTID *>(certid));
+  if (rv != 1) {
+    LOG(ERROR) << "OCSP_id_get0_info failed";
+    return -1;
+  }
+
+  if (serial == nullptr) {
+    LOG(ERROR) << "OCSP response does not contain serial number";
+    return -1;
+  }
+
+  auto cert = SSL_CTX_get0_certificate(ssl_ctx);
+  auto cert_serial = X509_get_serialNumber(cert);
+
+  if (ASN1_INTEGER_cmp(cert_serial, serial)) {
+    LOG(ERROR) << "OCSP verification serial numbers do not match";
+    return -1;
+  }
+
+  if (LOG_ENABLED(INFO)) {
+    LOG(INFO) << "OCSP verification succeeded";
+  }
+#endif // !defined(LIBRESSL_VERSION_NUMBER) && OPENSSL_VERSION_NUMBER >=
+       // 0x10002000L
+
+  return 0;
+}
+
 } // namespace tls
 
 } // namespace shrpx

src/shrpx_tls.h

@@ -264,6 +264,11 @@ X509 *load_certificate(const char *filename);
 // TLS version string.
 int proto_version_from_string(const StringRef &v);
 
+// Verifies OCSP response |ocsp_resp| of length |ocsp_resplen|.  This
+// function returns 0 if it succeeds, or -1.
+int verify_ocsp_response(SSL_CTX *ssl_ctx, const uint8_t *ocsp_resp,
+                         size_t ocsp_resplen);
+
 } // namespace tls
 
 } // namespace shrpx