[tls] Fix incorrect use of certificate verification callbacks (#35369)

As documented in [0], there are two certificate verification callbacks in the OpenSSL/BoringSSL TLS API. The one taken as a parameter to SSL_CTX_set_verify is the "verify callback". It is called multiple times during a single certificate verification is used to suppress errors and otherwise be notified about various events during verification.

Such a callback is not appropriate for accepting all certificates (you waste time processing things that will be thrown away), nor for post-verification inspection of the result (it will run multiple times). This is, however, what gRPC does with it.

Rather, gRPC should have used SSL_CTX_set_cert_verify_callback, which swaps out the verification process entirely. That is called exactly once per handshake and allows you to skip the verification, or verify and then inspect the results afterwards. Fix gRPC to heed the documentation.

In addition, this PR fixes a lifetime bug in gRPC's handling of the root certificate. RootCertExtractCallback stashes the root certificate without retaining it anywhere, but the X509_STORE_CTX will shortly be destroyed. There is no immediate guarantee the X509 object lasts as long as the SSL object. It most likely does because the object is often cached in the X509_STORE, which lives on the SSL_CTX, but this is at best, non-obvious. Instead, gRPC should have made
g_ssl_ex_verified_root_cert_index own a refcount to the X509 object by registering a free function and calling X509_up_ref when saving the value.

[0] https://commondatastorage.googleapis.com/chromium-boringssl-docs/ssl.h.html#SSL_CTX_set_verify

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Closes #35369

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35369 from davidben:wrong-verify-callback 5ccf3cf0f9
PiperOrigin-RevId: 597872521

src/core/tsi/ssl_transport_security.cc

@@ -176,6 +176,12 @@ static unsigned long openssl_thread_id_cb(void) {
 }
 #endif
 
+static void verified_root_cert_free(void* /*parent*/, void* ptr,
+                                    CRYPTO_EX_DATA* /*ad*/, int /*index*/,
+                                    long /*argl*/, void* /*argp*/) {
+  X509_free(static_cast<X509*>(ptr));
+}
+
 static void init_openssl(void) {
 #if OPENSSL_VERSION_NUMBER >= 0x10100000
   OPENSSL_init_ssl(0, nullptr);
@@ -207,8 +213,8 @@ static void init_openssl(void) {
       SSL_CTX_get_ex_new_index(0, nullptr, nullptr, nullptr, nullptr);
   GPR_ASSERT(g_ssl_ctx_ex_crl_provider_index != -1);
 
-  g_ssl_ex_verified_root_cert_index =
-      SSL_get_ex_new_index(0, nullptr, nullptr, nullptr, nullptr);
+  g_ssl_ex_verified_root_cert_index = SSL_get_ex_new_index(
+      0, nullptr, nullptr, nullptr, verified_root_cert_free);
   GPR_ASSERT(g_ssl_ex_verified_root_cert_index != -1);
 }
 
@@ -900,53 +906,40 @@ static int verify_cb(int ok, X509_STORE_CTX* ctx) {
 // the server's certificate, but we need to pull it anyway, in case a higher
 // layer wants to look at it. In this case the verification may fail, but
 // we don't really care.
-static int NullVerifyCallback(int /*preverify_ok*/, X509_STORE_CTX* /*ctx*/) {
+static int NullVerifyCallback(X509_STORE_CTX* /*ctx*/, void* /*arg*/) {
   return 1;
 }
 
-static int RootCertExtractCallback(int preverify_ok, X509_STORE_CTX* ctx) {
-  if (ctx == nullptr) {
-    return preverify_ok;
-  }
-
-  // There's a case where this function is set in SSL_CTX_set_verify and a CRL
-  // related callback is set with X509_STORE_set_verify_cb. They overlap and
-  // this will take precedence, thus we need to ensure the CRL related callback
-  // is still called
-  X509_VERIFY_PARAM* param = X509_STORE_CTX_get0_param(ctx);
-  auto flags = X509_VERIFY_PARAM_get_flags(param);
-  if (flags & X509_V_FLAG_CRL_CHECK) {
-    preverify_ok = verify_cb(preverify_ok, ctx);
-  }
-
-  // If preverify_ok == 0, verification failed. We shouldn't expect to have a
-  // verified chain, so there is no need to attempt to extract the root cert
-  // from it
-  if (preverify_ok == 0) {
-    return preverify_ok;
+static int RootCertExtractCallback(X509_STORE_CTX* ctx, void* /*arg*/) {
+  int ret = X509_verify_cert(ctx);
+  if (ret <= 0) {
+    // Verification failed. We shouldn't expect to have a verified chain, so
+    // there is no need to attempt to extract the root cert from it.
+    return ret;
   }
 
-  // If we're here, verification was successful
-  // Get the verified chain from the X509_STORE_CTX and put it on the SSL object
-  // so that we have access to it when populating the tsi_peer
+  // Verification was successful. Get the verified chain from the X509_STORE_CTX
+  // and put the root on the SSL object so that we have access to it when
+  // populating the tsi_peer. On error extracting the root, we return success
+  // anyway and proceed with the connection, to preserve the behavior of an
+  // older version of this code.
 #if OPENSSL_VERSION_NUMBER >= 0x10100000
   STACK_OF(X509)* chain = X509_STORE_CTX_get0_chain(ctx);
 #else
   STACK_OF(X509)* chain = X509_STORE_CTX_get_chain(ctx);
 #endif
-
   if (chain == nullptr) {
-    return preverify_ok;
+    return ret;
   }
 
   // The root cert is the last in the chain
   size_t chain_length = sk_X509_num(chain);
   if (chain_length == 0) {
-    return preverify_ok;
+    return ret;
   }
   X509* root_cert = sk_X509_value(chain, chain_length - 1);
   if (root_cert == nullptr) {
-    return preverify_ok;
+    return ret;
   }
 
   ERR_clear_error();
@@ -956,18 +949,32 @@ static int RootCertExtractCallback(int preverify_ok, X509_STORE_CTX* ctx) {
     ERR_error_string_n(ERR_get_error(), err_str, sizeof(err_str));
     gpr_log(GPR_ERROR,
             "error getting the SSL index from the X509_STORE_CTX: %s", err_str);
-    return preverify_ok;
+    return ret;
   }
   SSL* ssl = static_cast<SSL*>(X509_STORE_CTX_get_ex_data(ctx, ssl_index));
   if (ssl == nullptr) {
-    return preverify_ok;
+    return ret;
   }
+
+  // Free the old root and save the new one. There should not be an old root,
+  // but if renegotiation is not disabled (required by RFC 9113, Section
+  // 9.2.1), it is possible that this callback run multiple times for a single
+  // connection. gRPC does not always disable renegotiation. See
+  // https://github.com/grpc/grpc/issues/35368
+  X509_free(static_cast<X509*>(
+      SSL_get_ex_data(ssl, g_ssl_ex_verified_root_cert_index)));
   int success =
       SSL_set_ex_data(ssl, g_ssl_ex_verified_root_cert_index, root_cert);
   if (success == 0) {
     gpr_log(GPR_INFO, "Could not set verified root cert in SSL's ex_data");
+  } else {
+#if OPENSSL_VERSION_NUMBER >= 0x10100000L
+    X509_up_ref(root_cert);
+#else
+    CRYPTO_add(&root_cert->references, 1, CRYPTO_LOCK_X509);
+#endif
   }
-  return preverify_ok;
+  return ret;
 }
 
 // X509_STORE_set_get_crl() sets the function to get the crl for a given
@@ -2170,10 +2177,12 @@ tsi_result tsi_create_ssl_client_handshaker_factory_with_options(
     tsi_ssl_handshaker_factory_unref(&impl->base);
     return result;
   }
+  SSL_CTX_set_verify(ssl_context, SSL_VERIFY_PEER, nullptr);
   if (options->skip_server_certificate_verification) {
-    SSL_CTX_set_verify(ssl_context, SSL_VERIFY_PEER, NullVerifyCallback);
+    SSL_CTX_set_cert_verify_callback(ssl_context, NullVerifyCallback, nullptr);
   } else {
-    SSL_CTX_set_verify(ssl_context, SSL_VERIFY_PEER, RootCertExtractCallback);
+    SSL_CTX_set_cert_verify_callback(ssl_context, RootCertExtractCallback,
+                                     nullptr);
   }
 
 #if OPENSSL_VERSION_NUMBER >= 0x10100000
@@ -2355,22 +2364,28 @@ tsi_result tsi_create_ssl_server_handshaker_factory_with_options(
           SSL_CTX_set_verify(impl->ssl_contexts[i], SSL_VERIFY_NONE, nullptr);
           break;
         case TSI_REQUEST_CLIENT_CERTIFICATE_BUT_DONT_VERIFY:
-          SSL_CTX_set_verify(impl->ssl_contexts[i], SSL_VERIFY_PEER,
-                             NullVerifyCallback);
+          SSL_CTX_set_verify(impl->ssl_contexts[i], SSL_VERIFY_PEER, nullptr);
+          SSL_CTX_set_cert_verify_callback(impl->ssl_contexts[i],
+                                           NullVerifyCallback, nullptr);
           break;
         case TSI_REQUEST_CLIENT_CERTIFICATE_AND_VERIFY:
-          SSL_CTX_set_verify(impl->ssl_contexts[i], SSL_VERIFY_PEER,
-                             RootCertExtractCallback);
+          SSL_CTX_set_verify(impl->ssl_contexts[i], SSL_VERIFY_PEER, nullptr);
+          SSL_CTX_set_cert_verify_callback(impl->ssl_contexts[i],
+                                           RootCertExtractCallback, nullptr);
           break;
         case TSI_REQUEST_AND_REQUIRE_CLIENT_CERTIFICATE_BUT_DONT_VERIFY:
           SSL_CTX_set_verify(impl->ssl_contexts[i],
                              SSL_VERIFY_PEER | SSL_VERIFY_FAIL_IF_NO_PEER_CERT,
-                             NullVerifyCallback);
+                             nullptr);
+          SSL_CTX_set_cert_verify_callback(impl->ssl_contexts[i],
+                                           NullVerifyCallback, nullptr);
           break;
         case TSI_REQUEST_AND_REQUIRE_CLIENT_CERTIFICATE_AND_VERIFY:
           SSL_CTX_set_verify(impl->ssl_contexts[i],
                              SSL_VERIFY_PEER | SSL_VERIFY_FAIL_IF_NO_PEER_CERT,
-                             RootCertExtractCallback);
+                             nullptr);
+          SSL_CTX_set_cert_verify_callback(impl->ssl_contexts[i],
+                                           RootCertExtractCallback, nullptr);
           break;
       }