napi: fix memory corruption vulnerability

Fixes: https://hackerone.com/reports/784186
CVE-ID: CVE-2020-8174
PR-URL: nodejs-private/node-private#195
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: tniessen

src/js_native_api_v8.cc

@@ -2177,7 +2177,7 @@ napi_status napi_get_value_string_latin1(napi_env env,
   if (!buf) {
     CHECK_ARG(env, result);
     *result = val.As<v8::String>()->Length();
-  } else {
+  } else if (bufsize != 0) {
     int copied =
         val.As<v8::String>()->WriteOneByte(env->isolate,
                                            reinterpret_cast<uint8_t*>(buf),
@@ -2189,6 +2189,8 @@ napi_status napi_get_value_string_latin1(napi_env env,
     if (result != nullptr) {
       *result = copied;
     }
+  } else if (result != nullptr) {
+    *result = 0;
   }
 
   return napi_clear_last_error(env);
@@ -2216,7 +2218,7 @@ napi_status napi_get_value_string_utf8(napi_env env,
   if (!buf) {
     CHECK_ARG(env, result);
     *result = val.As<v8::String>()->Utf8Length(env->isolate);
-  } else {
+  } else if (bufsize != 0) {
     int copied = val.As<v8::String>()->WriteUtf8(
         env->isolate,
         buf,
@@ -2228,6 +2230,8 @@ napi_status napi_get_value_string_utf8(napi_env env,
     if (result != nullptr) {
       *result = copied;
     }
+  } else if (result != nullptr) {
+    *result = 0;
   }
 
   return napi_clear_last_error(env);
@@ -2256,7 +2260,7 @@ napi_status napi_get_value_string_utf16(napi_env env,
     CHECK_ARG(env, result);
     // V8 assumes UTF-16 length is the same as the number of characters.
     *result = val.As<v8::String>()->Length();
-  } else {
+  } else if (bufsize != 0) {
     int copied = val.As<v8::String>()->Write(env->isolate,
                                              reinterpret_cast<uint16_t*>(buf),
                                              0,
@@ -2267,6 +2271,8 @@ napi_status napi_get_value_string_utf16(napi_env env,
     if (result != nullptr) {
       *result = copied;
     }
+  } else if (result != nullptr) {
+    *result = 0;
   }
 
   return napi_clear_last_error(env);

test/js-native-api/test_string/test.js

@@ -81,3 +81,5 @@ assert.throws(() => {
 assert.throws(() => {
   test_string.TestLargeUtf16();
 }, /^Error: Invalid argument$/);
+
+test_string.TestMemoryCorruption(' '.repeat(64 * 1024));

test/js-native-api/test_string/test_string.c

@@ -1,4 +1,5 @@
 #include <limits.h>  // INT_MAX
+#include <string.h>
 #include <js_native_api.h>
 #include "../common.h"
 
@@ -244,6 +245,24 @@ static napi_value TestLargeUtf16(napi_env env, napi_callback_info info) {
   return output;
 }
 
+static napi_value TestMemoryCorruption(napi_env env, napi_callback_info info) {
+  size_t argc = 1;
+  napi_value args[1];
+  NAPI_CALL(env, napi_get_cb_info(env, info, &argc, args, NULL, NULL));
+
+  NAPI_ASSERT(env, argc == 1, "Wrong number of arguments");
+
+  char buf[10] = { 0 };
+  NAPI_CALL(env, napi_get_value_string_utf8(env, args[0], buf, 0, NULL));
+
+  char zero[10] = { 0 };
+  if (memcmp(buf, zero, sizeof(buf)) != 0) {
+    NAPI_CALL(env, napi_throw_error(env, NULL, "Buffer overwritten"));
+  }
+
+  return NULL;
+}
+
 EXTERN_C_START
 napi_value Init(napi_env env, napi_value exports) {
   napi_property_descriptor properties[] = {
@@ -258,6 +277,7 @@ napi_value Init(napi_env env, napi_value exports) {
     DECLARE_NAPI_PROPERTY("TestLargeUtf8", TestLargeUtf8),
     DECLARE_NAPI_PROPERTY("TestLargeLatin1", TestLargeLatin1),
     DECLARE_NAPI_PROPERTY("TestLargeUtf16", TestLargeUtf16),
+    DECLARE_NAPI_PROPERTY("TestMemoryCorruption", TestMemoryCorruption),
   };
 
   NAPI_CALL(env, napi_define_properties(