curl_multibyte: Remove local encoding fallbacks

- If the UTF-8 to UTF-16 conversion fails in Windows Unicode builds then
  no longer fall back to assuming the string is in a local encoding.

Background:

Some functions in Windows Unicode builds must convert UTF-8 to UTF-16 to
pass to the Windows CRT API wide-character functions since in Windows
UTF-8 is not a valid locale (or at least 99% of the time right now).

Prior to this change if the Unicode encoding conversion failed then
libcurl would assume, for backwards compatibility with applications that
may have written their code for non-Unicode builds, attempt to convert
the string from local encoding to UTF-16.

That type of "best effort" could theoretically cause some type of
security or other problem if a string that was locally encoded was also
valid UTF-8, and therefore an unexpected UTF-8 to UTF-16 conversion
could occur.

Ref: https://github.com/curl/curl/pull/7246

Closes https://github.com/curl/curl/pull/7257

1 files changed, 28 insertions, 22 deletions

diff --git a/lib/curl_multibyte.c b/lib/curl_multibyte.c
index 16418bee4..e9d2a8cb8 100644
--- a/lib/curl_multibyte.c
+++ b/lib/curl_multibyte.c
@@ -102,14 +102,16 @@ int curlx_win32_open(const char *filename, int oflag, ...)
   va_end(param);
 
 #ifdef _UNICODE
-  if(filename_w)
+  if(filename_w) {
     result = _wopen(filename_w, oflag, pmode);
-  free(filename_w);
-  if(result != -1)
-    return result;
-#endif
-
+    free(filename_w);
+  }
+  else
+    errno = EINVAL;
+  return result;
+#else
   return (_open)(filename, oflag, pmode);
+#endif
 }
 
 FILE *curlx_win32_fopen(const char *filename, const char *mode)
@@ -120,19 +122,20 @@ FILE *curlx_win32_fopen(const char *filename, const char *mode)
   wchar_t *mode_w = curlx_convert_UTF8_to_wchar(mode);
   if(filename_w && mode_w)
     result = _wfopen(filename_w, mode_w);
+  else
+    errno = EINVAL;
   free(filename_w);
   free(mode_w);
-  if(result)
-    return result;
-#endif
-
+  return result;
+#else
   return (fopen)(filename, mode);
+#endif
 }
 
 int curlx_win32_stat(const char *path, struct_stat *buffer)
 {
-  int result = -1;
 #ifdef _UNICODE
+  int result = -1;
   wchar_t *path_w = curlx_convert_UTF8_to_wchar(path);
   if(path_w) {
 #if defined(USE_WIN32_SMALL_FILES)
@@ -141,31 +144,34 @@ int curlx_win32_stat(const char *path, struct_stat *buffer)
     result = _wstati64(path_w, buffer);
 #endif
     free(path_w);
-    if(result != -1)
-      return result;
   }
-#endif /* _UNICODE */
-
+  else
+    errno = EINVAL;
+  return result;
+#else
 #if defined(USE_WIN32_SMALL_FILES)
-  result = _stat(path, buffer);
+  return _stat(path, buffer);
 #else
-  result = _stati64(path, buffer);
+  return _stati64(path, buffer);
+#endif
 #endif
-  return result;
 }
 
 int curlx_win32_access(const char *path, int mode)
 {
 #if defined(_UNICODE)
+  int result = -1;
   wchar_t *path_w = curlx_convert_UTF8_to_wchar(path);
   if(path_w) {
-    int result = _waccess(path_w, mode);
+    result = _waccess(path_w, mode);
     free(path_w);
-    if(result != -1)
-      return result;
   }
-#endif /* _UNICODE */
+  else
+    errno = EINVAL;
+  return result;
+#else
   return _access(path, mode);
+#endif
 }
 
 #endif /* USE_WIN32_LARGE_FILES || USE_WIN32_SMALL_FILES */