[3.5] bpo-33001: Prevent buffer overrun in os.symlink (GH-5989) (#5991)

* bpo-33001: Minimal fix to prevent buffer overrun in os.symlink

* Remove invalid test

1 files changed, 40 insertions, 0 deletions

diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index bb5d2e3..e207738 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -2117,6 +2117,46 @@ class Win32SymlinkTests(unittest.TestCase):
             os.remove(file1)
             shutil.rmtree(level1)
 
+    def test_buffer_overflow(self):
+        # Older versions would have a buffer overflow when detecting
+        # whether a link source was a directory. This test ensures we
+        # no longer crash, but does not otherwise validate the behavior
+        segment = 'X' * 27
+        path = os.path.join(*[segment] * 10)
+        test_cases = [
+            # overflow with absolute src
+            ('\\' + path, segment),
+            # overflow dest with relative src
+            (segment, path),
+            # overflow when joining src
+            (path[:180], path[:180]),
+        ]
+        for src, dest in test_cases:
+            try:
+                os.symlink(src, dest)
+            except FileNotFoundError:
+                pass
+            else:
+                try:
+                    os.remove(dest)
+                except OSError:
+                    pass
+            # Also test with bytes, since that is a separate code path.
+            # However, because of the stack layout, it is not possible
+            # to exploit the overflow on Python 3.5 using bytes
+            try:
+                os.symlink(os.fsencode(src), os.fsencode(dest))
+            except ValueError:
+                # Conversion function checks for len(arg) >= 260
+                pass
+            except FileNotFoundError:
+                pass
+            else:
+                try:
+                    os.remove(dest)
+                except OSError:
+                    pass
+
 
 @unittest.skipUnless(sys.platform == "win32", "Win32 specific tests")
 class Win32JunctionTests(unittest.TestCase):