Implement fuzzer mode for ECH server.

Now skipping over HPKE decryption in |ssl_client_hello_decrypt| when
fuzzer mode is enabled. To improve code coverage, this fuzzer-only logic
also also has the ability to simulate a failed decryption.

As a result of mostly skipping the decryption, we now have to exclude
"*-ECH-Server-Decline*" tests from running in fuzzer mode. These tests
rely on the now-broken assumption that decryption will fail when the
client used an ECHConfig unknown to the server.

Bug: 275
Change-Id: I759a79c8596897cdd3d3a37e05f2973d47346ef9
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/47624
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

ssl/encrypted_client_hello.cc

@@ -316,6 +316,20 @@ bool ssl_client_hello_decrypt(
     return false;
   }
 
+#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
+  // In fuzzer mode, disable encryption to improve coverage. We reserve a short
+  // input to signal decryption failure, so the fuzzer can explore fallback to
+  // ClientHelloOuter.
+  const uint8_t kBadPayload[] = {0xff};
+  if (payload == kBadPayload) {
+    *out_is_decrypt_error = true;
+    OPENSSL_PUT_ERROR(SSL, SSL_R_DECRYPTION_FAILED);
+    return false;
+  }
+  if (!out_encoded_client_hello_inner->CopyFrom(payload)) {
+    return false;
+  }
+#else
   // Attempt to decrypt into |out_encoded_client_hello_inner|.
   if (!out_encoded_client_hello_inner->Init(payload.size())) {
     OPENSSL_PUT_ERROR(SSL, ERR_R_MALLOC_FAILURE);
@@ -332,6 +346,7 @@ bool ssl_client_hello_decrypt(
     return false;
   }
   out_encoded_client_hello_inner->Shrink(encoded_client_hello_inner_len);
+#endif
   return true;
 }
 

ssl/test/fuzzer.h

@@ -230,6 +230,22 @@ const uint8_t kALPNProtocols[] = {
     0x01, 'a', 0x02, 'a', 'a', 0x03, 'a', 'a', 'a',
 };
 
+const uint8_t kECHServerConfig[] = {
+    0xfe, 0x0a, 0x00, 0x47, 0x2a, 0x00, 0x20, 0x00, 0x20, 0x6c, 0x55,
+    0x96, 0x41, 0x3d, 0x12, 0x4e, 0x63, 0x3d, 0x39, 0x7a, 0xe9, 0xbc,
+    0xec, 0xb2, 0x55, 0xd0, 0xe6, 0xaa, 0xbd, 0xa9, 0x79, 0xb8, 0x86,
+    0x9a, 0x13, 0x61, 0xc6, 0x69, 0xac, 0xb4, 0x21, 0x00, 0x0c, 0x00,
+    0x01, 0x00, 0x01, 0x00, 0x01, 0x00, 0x02, 0x00, 0x01, 0x00, 0x03,
+    0x00, 0x10, 0x00, 0x0e, 0x70, 0x75, 0x62, 0x6c, 0x69, 0x63, 0x2e,
+    0x65, 0x78, 0x61, 0x6d, 0x70, 0x6c, 0x65, 0x00, 0x00,
+};
+
+const uint8_t kECHServerConfigPrivateKey[] = {
+    0x35, 0x6d, 0x45, 0x06, 0xb3, 0x88, 0x89, 0x2e, 0xd6, 0x87, 0x84,
+    0xd2, 0x2d, 0x6f, 0x83, 0x48, 0xad, 0xf2, 0xfd, 0x08, 0x51, 0x73,
+    0x10, 0xa0, 0xb8, 0xdd, 0xe9, 0x96, 0x6a, 0xde, 0xbc, 0x82,
+};
+
 int ALPNSelectCallback(SSL *ssl, const uint8_t **out, uint8_t *out_len,
                        const uint8_t *in, unsigned in_len, void *arg) {
   static const uint8_t kProtocol[] = {'a', 'a'};
@@ -438,6 +454,23 @@ class TLSFuzzer {
     }
     SSL_CTX_set_tls_channel_id_enabled(ctx_.get(), 1);
 
+    if (role_ == kServer) {
+      bssl::UniquePtr<SSL_ECH_SERVER_CONFIG_LIST> config_list(
+          SSL_ECH_SERVER_CONFIG_LIST_new());
+      if (!config_list) {
+        return false;
+      }
+      if (!SSL_ECH_SERVER_CONFIG_LIST_add(
+              config_list.get(), /*is_retry_config=*/true, kECHServerConfig,
+              sizeof(kECHServerConfig), kECHServerConfigPrivateKey,
+              sizeof(kECHServerConfigPrivateKey))) {
+        return false;
+      }
+      if (!SSL_CTX_set1_ech_server_config_list(ctx_.get(), config_list.get())) {
+        return false;
+      }
+    }
+
     return true;
   }
 

ssl/test/fuzzer_tags.h

@@ -23,7 +23,7 @@
 // The TLS client and server fuzzers coordinate with bssl_shim on a common
 // format to encode configuration parameters in a fuzzer file. To add a new
 // configuration, define a tag, update |SetupTest| in fuzzer.h to parse it, and
-// update |WriteSettings| in bssl_shim to serialize it. Finally, record
+// update |SettingsWriter| in bssl_shim to serialize it. Finally, record
 // transcripts from a test run, and use the BORINGSSL_FUZZER_DEBUG environment
 // variable to confirm the transcripts are compatible.
 

ssl/test/runner/fuzzer_mode.json

@@ -51,6 +51,8 @@
 
     "Renegotiate-Client-BadExt*": "Fuzzer mode does not check renegotiation_info.",
 
-    "CBCRecordSplitting*": "Fuzzer mode does not implement record-splitting."
+    "CBCRecordSplitting*": "Fuzzer mode does not implement record-splitting.",
+
+    "*-ECH-Server-Decline*": "Encryption with wrong ECHConfig will not fail because fuzzer mode skips HPKE decryption."
   }
 }

ssl/test/runner/handshake_client.go

@@ -23,6 +23,8 @@ import (
 	"boringssl.googlesource.com/boringssl/ssl/test/runner/hpke"
 )
 
+const echBadPayloadByte = 0xff
+
 type clientHandshakeState struct {
 	c                 *Conn
 	serverHello       *serverHelloMsg
@@ -826,9 +828,13 @@ NextCipherSuite:
 			return nil, err
 		}
 		if c.config.Bugs.CorruptEncryptedClientHello {
+			if c.config.Bugs.NullAllCiphers {
+				hello.clientECH.payload = []byte{echBadPayloadByte}
+			} else {
 				hello.clientECH.payload[0] ^= 1
 			}
 		}
+	}
 
 	// PSK binders and ECH both must be computed last because they incorporate
 	// the rest of the ClientHello and conflict. ECH resolves this by forbidding
@@ -884,6 +890,10 @@ func (hs *clientHandshakeState) encryptClientHello(hello, innerHello *clientHell
 	encodedInner := innerHello.marshalForEncodedInner()
 	payload := hs.echHPKEContext.Seal(encodedInner, aad.finish())
 
+	if c.config.Bugs.NullAllCiphers {
+		payload = encodedInner
+	}
+
 	// Place the ECH extension in the outer CH.
 	hello.clientECH = &clientECH{
 		hpkeKDF:  hs.echHPKEContext.KDF(),
@@ -1534,10 +1544,14 @@ func (hs *clientHandshakeState) applyHelloRetryRequest(helloRetryRequest *helloR
 				return err
 			}
 			if c.config.Bugs.CorruptSecondEncryptedClientHello {
+				if c.config.Bugs.NullAllCiphers {
+					hello.clientECH.payload = []byte{echBadPayloadByte}
+				} else {
 					hello.clientECH.payload[0] ^= 1
 				}
 			}
 		}
+	}
 
 	// PSK binders and ECH both must be inserted last because they incorporate
 	// the rest of the ClientHello and conflict. See corresponding comment in