Rewrite symlink sanitization logic

We can rewrite symlinks to ensure they are always relative
and remain within the extraction directory.

Author: AndrewFasano

unblob/extractor.py

@@ -47,68 +47,73 @@ def is_recursive_link(path: Path) -> bool:
     return False
 
 
-def fix_symlink(path: Path, outdir: Path, task_result: TaskResult) -> Path:
-    """Rewrites absolute symlinks to point within the extraction directory (outdir).
-
-    If it's not a relative symlink it is either removed it it attempts
-    to traverse outside of the extraction directory or rewritten to be
-    fully portable (no mention of the extraction directory in the link
-    value).
-    """
-    if is_recursive_link(path):
-        logger.error("Symlink loop identified, removing", path=path)
-        error_report = MaliciousSymlinkRemoved(
-            link=path.as_posix(), target=os.readlink(path)
-        )
-        task_result.add_report(error_report)
-        path.unlink()
-        return path
-
-    raw_target = os.readlink(path)
-    if not raw_target:
-        logger.error("Symlink with empty target, removing.")
-        path.unlink()
-        return path
-
-    target = Path(raw_target)
-
-    if target.is_absolute():
-        target = Path(target.as_posix().lstrip("/"))
+def sanitize_symlink_target(base_dir, current_dir, target):
+    # Normalize all paths to their absolute forms
+    base_dir_abs = os.path.abspath(base_dir)
+    current_dir_abs = os.path.abspath(current_dir)
+    target_abs = os.path.abspath(os.path.join(current_dir, target)) \
+                    if not os.path.isabs(target) else os.path.abspath(target)
+
+    # Check if the target is absolute and within the base_dir
+    if os.path.isabs(target):
+        if target_abs.startswith(base_dir_abs):
+            return os.path.relpath(target_abs, current_dir_abs)
+        else:
+            # Target is absolute but outside base_dir - we'll pretend base_dir is our root
+            # and adjust the target to be within base_dir
+            abs = base_dir + "/" + os.path.relpath(target_abs, os.path.commonpath([target_abs, base_dir_abs]))
+            # We want to return the relative path from current_dir to the adjusted target
+            return os.path.relpath(abs, current_dir_abs)
     else:
-        target = path.resolve()
-
-    safe = is_safe_path(outdir, target)
-
-    if not safe:
-        logger.error("Path traversal attempt through symlink, removing", target=target)
-        error_report = MaliciousSymlinkRemoved(
-            link=path.as_posix(), target=target.as_posix()
-        )
-        task_result.add_report(error_report)
-        path.unlink()
-    else:
-        relative_target = os.path.relpath(outdir.joinpath(target), start=path.parent)
-        path.unlink()
-        path.symlink_to(relative_target)
-    return path
-
+        # Target is relative
+        if target_abs.startswith(base_dir_abs):
+            # Target is relative and does not escape base_dir
+            return os.path.relpath(target_abs, current_dir_abs)
+        else:
+            # Target is relative and escapes base_dir
+            # Say we have foo/passwd -> ../../../etc/passwd with root at /host/test_archive
+            # from /host/test_archive/foo/passwd, we want to return ../etc/passwd which is the
+            # relative path from /host/test_archive/foo to /host/test_archive/etc/passwd
+            # without escaping /host/test_archive
+
+            for drop_count in range(0, len(target.split('/'))):
+                # We drop '..'s from the target by prepending placeholder directories until we get something valid
+                abs = current_dir + "/" + "/".join(["foo"] * drop_count) + target
+                resolved = os.path.abspath(abs)
+                if resolved.startswith(base_dir_abs):
+                    break
+            else:
+                raise ValueError(f"Could not resolve symlink target {target} within base_dir {base_dir}")
+
+            # We need to add the /placeholder to the relative path because we need
+            # to act like a file within base_dir is our root (as opposed to base_dir itself)
+            return os.path.relpath(resolved, base_dir_abs + '/placeholder')
 
 def fix_extracted_directory(outdir: Path, task_result: TaskResult):
     def _fix_extracted_directory(directory: Path):
         if not directory.exists():
             return
-        for path in (directory / p for p in os.listdir(directory)):
-            try:
-                fix_permission(path)
-                if path.is_symlink():
-                    fix_symlink(path, outdir, task_result)
-                    continue
-                if path.is_dir():
-                    _fix_extracted_directory(path)
-            except OSError as e:
-                if e.errno == errno.ENAMETOOLONG:
-                    continue
-                raise e from None
+
+        base_dir = os.path.abspath(outdir)
+        for root, dirs, files in os.walk(base_dir, topdown=True):
+            fix_permission(Path(root))
+            for name in dirs+files:
+                try:
+                    full_path = os.path.join(root, name)
+                    if os.path.islink(full_path):
+                        # Make symlinks relative and constrain them to the base_dir
+                        target = os.readlink(full_path)
+                        new_target = sanitize_symlink_target(base_dir, root, target)
+                        if new_target != target:
+                            os.remove(full_path)
+                            os.symlink(new_target, full_path)
+                            logger.info("Updated symlink", path=full_path, target=new_target)
+                        else:
+                            logger.debug("Symlink is already sanitized", path=full_path, target=new_target)
+                except OSError as e:
+                    if e.errno == errno.ENAMETOOLONG:
+                        continue
+                    raise e from None
 
     fix_permission(outdir)
     _fix_extracted_directory(outdir)