Manually resolves links relatively to root_dir, and prevent escape

This patch is a followup of #311.

It appeared that we were not resolving paths when reading from files.
This means that a symbolic link present under `root_dir` could be
blindly followed _outside_ of `root_dir`, possibly leading to host
files.

Author: Samuel FORESTIER

src/distro/distro.py

@@ -757,7 +757,7 @@ def __init__(
         * :py:exc:`UnicodeError`: A data source has unexpected characters or
           uses an unexpected encoding.
         """
-        self.root_dir = root_dir
+        self.root_dir = os.path.realpath(root_dir) if root_dir else None
         self.etc_dir = os.path.join(root_dir, "etc") if root_dir else _UNIXCONFDIR
         self.usr_lib_dir = (
             os.path.join(root_dir, "usr/lib") if root_dir else _UNIXUSRLIBDIR
@@ -773,8 +773,10 @@ def __init__(
 
             # NOTE: The idea is to respect order **and** have it set
             #       at all times for API backwards compatibility.
-            if os.path.isfile(etc_dir_os_release_file) or not os.path.isfile(
-                usr_lib_os_release_file
+            if (
+                self.root_dir is not None
+                or os.path.isfile(etc_dir_os_release_file)
+                or not os.path.isfile(usr_lib_os_release_file)
             ):
                 self.os_release_file = etc_dir_os_release_file
             else:
@@ -1091,6 +1093,66 @@ def uname_attr(self, attribute: str) -> str:
         """
         return self._uname_info.get(attribute, "")
 
+    @staticmethod
+    def __abs_path_join(root_path: str, abs_path: str) -> str:
+        rel_path = os.path.splitdrive(abs_path)[1].lstrip(os.sep)
+        if os.altsep is not None:
+            rel_path = rel_path.lstrip(os.altsep)
+
+        return os.path.join(root_path, rel_path)
+
+    def __resolve_chroot_symlink_as_needed(self, link_location: str) -> str:
+        """
+        Resolves a potential symlink in ``link_location`` against
+        ``self.root_dir`` if inside the chroot, else just return the original
+        path.
+        We're doing this check at a central place, to making the calling code
+        more readable and to de-duplicate.
+        """
+        if self.root_dir is None:
+            return link_location
+
+        # consider non-absolute `link_location` relative to `root_dir` (as
+        # `os.path.commonpath` does not support mixing absolute and relative
+        # paths).
+        if not os.path.isabs(link_location):
+            link_location = self.__abs_path_join(self.root_dir, link_location)
+
+        seen_paths = set()
+        while True:
+            # while `link_location` _should_ be relative to chroot (either
+            # passed from trusted code or already resolved by previous loop
+            # iteration), we enforce this check as `self.os_release_file` and
+            # `self.distro_release_file` may be user-supplied.
+            if os.path.commonpath(
+                [self.root_dir, link_location]
+            ) != self.root_dir.rstrip(os.sep):
+                raise FileNotFoundError
+
+            if not os.path.islink(link_location):
+                return link_location
+
+            resolved = os.readlink(link_location)
+            if not os.path.isabs(resolved):
+                # compute resolved path relatively to previous `link_location`
+                # and accordingly to chroot. We also canonize "top" `..`
+                # components (relatively to `self.root_dir`), as they would
+                # legitimately resolve to chroot itself).
+                resolved = os.path.relpath(
+                    os.path.join(os.path.dirname(link_location), resolved),
+                    start=self.root_dir,
+                ).lstrip(os.pardir + os.pathsep)
+
+            # "move" back absolute path inside the chroot
+            resolved = self.__abs_path_join(self.root_dir, resolved)
+
+            # prevent symlinks infinite loop
+            if resolved in seen_paths:
+                raise FileNotFoundError
+
+            seen_paths.add(link_location)
+            link_location = resolved
+
     @cached_property
     def _os_release_info(self) -> Dict[str, str]:
         """
@@ -1099,10 +1161,14 @@ def _os_release_info(self) -> Dict[str, str]:
         Returns:
             A dictionary containing all information items.
         """
-        if os.path.isfile(self.os_release_file):
-            with open(self.os_release_file, encoding="utf-8") as release_file:
+        try:
+            with open(
+                self.__resolve_chroot_symlink_as_needed(self.os_release_file),
+                encoding="utf-8",
+            ) as release_file:
                 return self._parse_os_release_content(release_file)
-        return {}
+        except OSError:
+            return {}
 
     @staticmethod
     def _parse_os_release_content(lines: TextIO) -> Dict[str, str]:
@@ -1223,7 +1289,10 @@ def _oslevel_info(self) -> str:
     def _debian_version(self) -> str:
         try:
             with open(
-                os.path.join(self.etc_dir, "debian_version"), encoding="ascii"
+                self.__resolve_chroot_symlink_as_needed(
+                    os.path.join(self.etc_dir, "debian_version")
+                ),
+                encoding="ascii",
             ) as fp:
                 return fp.readline().rstrip()
         except FileNotFoundError:
@@ -1233,7 +1302,10 @@ def _debian_version(self) -> str:
     def _armbian_version(self) -> str:
         try:
             with open(
-                os.path.join(self.etc_dir, "armbian-release"), encoding="ascii"
+                self.__resolve_chroot_symlink_as_needed(
+                    os.path.join(self.etc_dir, "armbian-release")
+                ),
+                encoding="ascii",
             ) as fp:
                 return self._parse_os_release_content(fp).get("version", "")
         except FileNotFoundError:
@@ -1285,9 +1357,10 @@ def _distro_release_info(self) -> Dict[str, str]:
             try:
                 basenames = [
                     basename
-                    for basename in os.listdir(self.etc_dir)
+                    for basename in os.listdir(
+                        self.__resolve_chroot_symlink_as_needed(self.etc_dir)
+                    )
                     if basename not in _DISTRO_RELEASE_IGNORE_BASENAMES
-                    and os.path.isfile(os.path.join(self.etc_dir, basename))
                 ]
                 # We sort for repeatability in cases where there are multiple
                 # distro specific files; e.g. CentOS, Oracle, Enterprise all
@@ -1303,12 +1376,13 @@ def _distro_release_info(self) -> Dict[str, str]:
                 match = _DISTRO_RELEASE_BASENAME_PATTERN.match(basename)
                 if match is None:
                     continue
-                filepath = os.path.join(self.etc_dir, basename)
-                distro_info = self._parse_distro_release_file(filepath)
+                # NOTE: _parse_distro_release_file below will be resolving for us
+                unresolved_filepath = os.path.join(self.etc_dir, basename)
+                distro_info = self._parse_distro_release_file(unresolved_filepath)
                 # The name is always present if the pattern matches.
                 if "name" not in distro_info:
                     continue
-                self.distro_release_file = filepath
+                self.distro_release_file = unresolved_filepath
                 break
             else:  # the loop didn't "break": no candidate.
                 return {}
@@ -1342,7 +1416,9 @@ def _parse_distro_release_file(self, filepath: str) -> Dict[str, str]:
             A dictionary containing all information items.
         """
         try:
-            with open(filepath, encoding="utf-8") as fp:
+            with open(
+                self.__resolve_chroot_symlink_as_needed(filepath), encoding="utf-8"
+            ) as fp:
                 # Only parse the first line. For instance, on SLES there
                 # are multiple lines. We don't want them...
                 return self._parse_distro_release_content(fp.readline())

tests/resources/testdistros/distro/root_dir_absolute_symlinks/etc/os-release

@@ -0,0 +1 @@
+/tmp/another-link

tests/resources/testdistros/distro/root_dir_absolute_symlinks/tmp/another-link

@@ -0,0 +1 @@
+/usr/lib/os-release

tests/resources/testdistros/distro/root_dir_absolute_symlinks/usr/lib/os-release

@@ -0,0 +1 @@
+ID=absolute_symlinks

tests/resources/testdistros/distro/root_dir_escape/etc/os-release

@@ -0,0 +1 @@
+../../../../distros/debian8/etc/os-release

tests/resources/testdistros/distro/root_dir_non_escape/etc/os-release

@@ -0,0 +1 @@
+/tmp/another-link/../../../usr/lib/os-release

tests/resources/testdistros/distro/root_dir_non_escape/tmp/another-link

@@ -0,0 +1 @@
+nested/nested

tests/resources/testdistros/distro/root_dir_non_escape/tmp/nested/nested/.gitkeep

@@ -0,0 +1 @@
+ID=root_dir_non_escape

tests/resources/testdistros/distro/root_dir_non_escape/usr/lib/os-release

@@ -0,0 +1 @@
+ID=root_dir_os_release_file_abs