[3.5] bpo-29778: Ensure python3.dll is loaded from correct locations when Python is embedded (GH-21297) (#21377)

bpo-29778: Ensure python3.dll is loaded from correct locations when Python is embedded.

Author: zooba

Misc/NEWS.d/next/Security/2020-07-03-17-21-37.bpo-29778.cR_fGS.rst

@@ -0,0 +1,2 @@
+Ensure :file:`python3.dll` is loaded from correct locations when Python is
+embedded (CVE-2020-15523).

PC/getpathp.c

@@ -93,7 +93,6 @@
 
 static wchar_t prefix[MAXPATHLEN+1];
 static wchar_t progpath[MAXPATHLEN+1];
-static wchar_t dllpath[MAXPATHLEN+1];
 static wchar_t *module_search_path = NULL;
 
 
@@ -122,6 +121,46 @@ reduce(wchar_t *dir)
  dir[i] = '\0';
 }
 
+static int
+change_ext(wchar_t *dest, const wchar_t *src, const wchar_t *ext)
+{
+ if (src && src != dest) {
+ size_t src_len = wcsnlen_s(src, MAXPATHLEN+1);
+ size_t i = src_len;
+ if (i >= MAXPATHLEN+1) {
+ Py_FatalError("buffer overflow in getpathp.c's reduce()");
+ }
+
+ while (i > 0 && src[i] != '.' && !is_sep(src[i]))
+ --i;
+
+ if (i == 0) {
+ dest[0] = '\0';
+ return -1;
+ }
+
+ if (is_sep(src[i])) {
+ i = src_len;
+ }
+
+ if (wcsncpy_s(dest, MAXPATHLEN+1, src, i)) {
+ dest[0] = '\0';
+ return -1;
+ }
+ } else {
+ wchar_t *s = wcsrchr(dest, L'.');
+ if (s) {
+ s[0] = '\0';
+ }
+ }
+
+ if (wcscat_s(dest, MAXPATHLEN+1, ext)) {
+ dest[0] = '\0';
+ return -1;
+ }
+
+ return 0;
+}
 
 static int
 exists(wchar_t *filename)
@@ -214,6 +253,20 @@ search_for_prefix(wchar_t *argv0_path, wchar_t *landmark)
  return 0;
 }
 
+
+static int
+get_dllpath(wchar_t *dllpath)
+{
+#ifdef Py_ENABLE_SHARED
+ extern HANDLE PyWin_DLLhModule;
+ if (PyWin_DLLhModule && GetModuleFileNameW(PyWin_DLLhModule, dllpath, MAXPATHLEN)) {
+ return 0;
+ }
+#endif
+ return -1;
+}
+
+
 #ifdef MS_WINDOWS
 #ifdef Py_ENABLE_SHARED
 
@@ -378,19 +431,8 @@ get_progpath(void)
  wchar_t *path = _wgetenv(L"PATH");
  wchar_t *prog = Py_GetProgramName();
 
-#ifdef MS_WINDOWS
-#ifdef Py_ENABLE_SHARED
- extern HANDLE PyWin_DLLhModule;
- /* static init of progpath ensures final char remains \0 */
- if (PyWin_DLLhModule)
- if (!GetModuleFileNameW(PyWin_DLLhModule, dllpath, MAXPATHLEN))
- dllpath[0] = 0;
-#else
- dllpath[0] = 0;
-#endif
  if (GetModuleFileNameW(NULL, progpath, MAXPATHLEN))
  return;
-#endif
  if (prog == NULL || *prog == '\0')
  prog = L"python";
 
@@ -578,14 +620,10 @@ calculate_path(void)
 
 #ifdef MS_WINDOWS
  /* Calculate zip archive path from DLL or exe path */
- if (wcscpy_s(zip_path, MAXPATHLEN+1, dllpath[0] ? dllpath : progpath))
- /* exceeded buffer length - ignore zip_path */
- zip_path[0] = '\0';
- else {
- wchar_t *dot = wcsrchr(zip_path, '.');
- if (!dot || wcscpy_s(dot, MAXPATHLEN+1 - (dot - zip_path), L".zip"))
- /* exceeded buffer length - ignore zip_path */
- zip_path[0] = L'\0';
+ if (!get_dllpath(zip_path)) {
+ change_ext(zip_path, zip_path, L".zip");
+ } else {
+ change_ext(zip_path, progpath, L".zip");
  }
 
  skiphome = pythonhome==NULL ? 0 : 1;
@@ -768,8 +806,6 @@ calculate_path(void)
 }
 
 
-/* External interface */
-
 void
 Py_SetPath(const wchar_t *path)
 {
@@ -830,25 +866,39 @@ int
 _Py_CheckPython3()
 {
  wchar_t py3path[MAXPATHLEN+1];
- wchar_t *s;
- if (python3_checked)
+ if (python3_checked) {
  return hPython3 != NULL;
+ }
  python3_checked = 1;
 
  /* If there is a python3.dll next to the python3y.dll,
- assume this is a build tree; use that DLL */
- wcscpy(py3path, dllpath);
- s = wcsrchr(py3path, L'\\');
- if (!s)
- s = py3path;
- wcscpy(s, L"\\python3.dll");
- hPython3 = LoadLibraryExW(py3path, NULL, LOAD_WITH_ALTERED_SEARCH_PATH);
- if (hPython3 != NULL)
-  return 1;
+ use that DLL */
+ if (!get_dllpath(py3path)) {
+  reduce(py3path);
+  join(py3path, PY3_DLLNAME);
+ hPython3 = LoadLibraryExW(py3path, NULL, LOAD_WITH_ALTERED_SEARCH_PATH);
+  if (hPython3 != NULL) {
+  return 1;
+  }
+ }
 
- /* Check sys.prefix\DLLs\python3.dll */
+ /* If we can locate python3.dll in our application dir,
+ use that DLL */
  wcscpy(py3path, Py_GetPrefix());
- wcscat(py3path, L"\\DLLs\\python3.dll");
- hPython3 = LoadLibraryExW(py3path, NULL, LOAD_WITH_ALTERED_SEARCH_PATH);
+ if (py3path[0]) {
+ join(py3path, PY3_DLLNAME);
+ hPython3 = LoadLibraryExW(py3path, NULL, LOAD_WITH_ALTERED_SEARCH_PATH);
+ if (hPython3 != NULL) {
+ return 1;
+ }
+ }
+
+ /* For back-compat, also search {sys.prefix}\DLLs, though
+ that has not been a normal install layout for a while */
+ wcscpy(py3path, Py_GetPrefix());
+ if (py3path[0]) {
+ join(py3path, L"DLLs\\" PY3_DLLNAME);
+ hPython3 = LoadLibraryExW(py3path, NULL, LOAD_WITH_ALTERED_SEARCH_PATH);
+ }
  return hPython3 != NULL;
 }

PCbuild/pyproject.props

@@ -24,12 +24,12 @@
  <_PlatformPreprocessorDefinition>_WIN32;</_PlatformPreprocessorDefinition>
  <_PlatformPreprocessorDefinition Condition="$(Platform) == 'x64'">_WIN64;_M_X64;</_PlatformPreprocessorDefinition>
  <_PydPreprocessorDefinition Condition="$(TargetExt) == '.pyd'">Py_BUILD_CORE_MODULE;</_PydPreprocessorDefinition>
+ <_Py3NamePreprocessorDefinition>PY3_DLLNAME=L"$(Py3DllName)";</_Py3NamePreprocessorDefinition>
  </PropertyGroup>
  <ItemDefinitionGroup>
  <ClCompile>
  <AdditionalIncludeDirectories>$(PySourcePath)Include;$(PySourcePath)PC;$(IntDir);%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
- <PreprocessorDefinitions>WIN32;$(_PlatformPreprocessorDefinition)$(_DebugPreprocessorDefinition)$(_PydPreprocessorDefinition)%(PreprocessorDefinitions)</PreprocessorDefinitions>
- 
+ <PreprocessorDefinitions>WIN32;$(_Py3NamePreprocessorDefinition)$(_PlatformPreprocessorDefinition)$(_DebugPreprocessorDefinition)$(_PydPreprocessorDefinition)%(PreprocessorDefinitions)</PreprocessorDefinitions>
  <Optimization>MaxSpeed</Optimization>
  <IntrinsicFunctions>true</IntrinsicFunctions>
  <StringPooling>true</StringPooling>

PCbuild/python.props

@@ -152,6 +152,8 @@
 
  <!-- The name of the resulting pythonXY.dll (without the extension) -->
  <PyDllName>python$(MajorVersionNumber)$(MinorVersionNumber)$(PyDebugExt)</PyDllName>
+ <!-- The name of the resulting pythonX.dll (without the extension) -->
+ <Py3DllName>python3$(PyDebugExt)</Py3DllName>
 
  <!-- The version and platform tag to include in .pyd filenames -->
  <PydTag Condition="$(ArchName) == 'win32'">.cp$(MajorVersionNumber)$(MinorVersionNumber)-win32</PydTag>

Python/dynload_win.c

@@ -194,9 +194,7 @@ dl_funcptr _PyImport_FindSharedFuncptrWindows(const char *prefix,
  char funcname[258], *import_python;
  const wchar_t *wpathname;
 
-#ifndef _DEBUG
  _Py_CheckPython3();
-#endif
 
  wpathname = _PyUnicode_AsUnicode(pathname);
  if (wpathname == NULL)