Forbid extracting files with absolute path from 'ar' archives (boo#1122683)

Also fix and modernize the code, add tests.

osc/util/ar.py

@@ -19,36 +19,32 @@ import re
 import stat
 import sys
 from io import BytesIO
-
-
-# workaround for python24
-if not hasattr(os, 'SEEK_SET'):
-    os.SEEK_SET = 0
+from typing import Union
 
 
 class ArError(Exception):
     """Base class for all ar related errors"""
 
-    def __init__(self, fn, msg):
+    def __init__(self, fn: bytes, msg: str):
         super().__init__()
         self.file = fn
         self.msg = msg
 
     def __str__(self):
-        return 'ar error: %s' % self.msg
+        return f"{self.msg}: {self.file.decode('utf-8')}"
 
 
 class ArHdr:
     """Represents an ar header entry"""
 
-    def __init__(self, fn, date, uid, gid, mode, size, fmag, off):
+    def __init__(self, fn: bytes, date: bytes, uid: bytes, gid: bytes, mode: bytes, size: bytes, fmag: bytes, off: bytes):
         self.file = fn.strip()
         self.date = date.strip()
         self.uid = uid.strip()
         self.gid = gid.strip()
         if not mode.strip():
             # provide a dummy mode for the ext_fn hdr
-            mode = '0'
+            mode = b"0"
         self.mode = stat.S_IMODE(int(mode, 8))
         self.size = int(size)
         self.fmag = fmag
@@ -75,9 +71,17 @@ class ArFile(BytesIO):
         working dir is used. Additionally it tries to set the owner/group
         and permissions.
         """
+        if self.name.startswith(b"/"):
+            raise ArError(self.name, "Extracting files with absolute paths is not supported for security reasons")
+
         if not dir:
             dir = os.getcwdb()
-        fn = os.path.join(dir, self.name)
+        fn = os.path.join(dir, self.name.decode("utf-8"))
+
+        dir_path, _ = os.path.split(fn)
+        if dir_path:
+            os.makedirs(dir_path, exist_ok=True)
+
         with open(fn, 'wb') as f:
             f.write(self.getvalue())
         os.chmod(fn, self.mode)
@@ -88,6 +92,7 @@ class ArFile(BytesIO):
         if gid not in os.getgroups() or os.getegid() != 0:
             gid = -1
         os.chown(fn, uid, gid)
+        return fn
 
     def __str__(self):
         return '%s %s %s %s' % (self.name, self.uid,
@@ -140,26 +145,30 @@ class Ar:
         data section. The end of such a filename is indicated with a trailing '/'.
         Another special file is the '/' which contains the symbol lookup table.
         """
+
+        # read extended header with long file names and then only seek into the right offsets
+        self.__file.seek(self.ext_fnhdr.dataoff, os.SEEK_SET)
+        ext_fnhdr_data = self.__file.read(self.ext_fnhdr.size)
+
         for h in self.hdrs:
             if h.file == b'/':
                 continue
-            # remove slashes which are appended by ar
-            h.file = h.file.rstrip(b'/')
-            if not h.file.startswith(b'/'):
+
+            if h.file.endswith(b"/"):
+                # regular file name
+                h.file = h.file[:-1]
                 continue
-            # handle long filename
-            off = int(h.file[1:len(h.file)])
-            start = self.ext_fnhdr.dataoff + off
-            self.__file.seek(start, os.SEEK_SET)
-            # XXX: is it safe to read all the data in one chunk? I assume the '//' data section
-            #      won't be too large
-            data = self.__file.read(self.ext_fnhdr.size)
-            end = data.find(b'/')
-            if end != -1:
-                h.file = data[0:end]
-            else:
+
+            # long file name
+            assert h.file[0:1] == b"/"
+            start = int(h.file[1:])
+            end = ext_fnhdr_data.find(b'/', start)
+
+            if end == -1:
                 raise ArError(b'//', 'invalid data section - trailing slash (off: %d)' % start)
 
+            h.file = ext_fnhdr_data[start:end]
+
     def _get_file(self, hdr):
         self.__file.seek(hdr.dataoff, os.SEEK_SET)
         return ArFile(hdr.file, hdr.uid, hdr.gid, hdr.mode,
@@ -193,7 +202,10 @@ class Ar:
             pos += hdr.size + (hdr.size & 1)
         self._fixupFilenames()
 
-    def get_file(self, fn):
+    def get_file(self, fn: Union[str, bytes]):
+        # accept str for better user experience
+        if isinstance(fn, str):
+            fn = fn.encode("utf-8")
         for h in self.hdrs:
             if h.file == fn:
                 return self._get_file(h)

tests/fixtures/README

@@ -0,0 +1,28 @@
+Generate data for creating test archives
+----------------------------------------
+
+echo 'foobar' > /tmp/foo
+
+# root perms required for the next command
+echo 'numbers' > /123
+
+echo 'qwerty' > very-long-long-long-long-name
+
+echo 'asdfgh' > very-long-long-long-long-name2
+
+echo 'newline' > 'very-long-name
+-with-newline'
+
+echo 'newline' > 'a
+b'
+
+mkdir 'dir'
+echo 'file-in-a-dir' > dir/file
+
+
+Create archive.ar
+-----------------
+
+ar qP archive.ar /tmp/foo /123 very-long-long-long-long-name very-long-long-long-long-name2 'very-long-name
+-with-newline' 'a
+b' dir/file

tests/fixtures/archive.ar

@@ -0,0 +1,25 @@
+!<arch>
+//                                              94        `
+very-long-long-long-long-name/
+very-long-long-long-long-name2/
+very-long-name
+-with-newline/
+
+/tmp/foo/       1716888536  1000  1000  100644  7         `
+foobar
+
+/123/           1716883776  0     0     100644  8         `
+numbers
+/0              1716882802  1000  1000  100644  7         `
+qwerty
+
+/31             1716882988  1000  1000  100644  7         `
+asdfgh
+
+/63             1716884767  1000  1000  100644  8         `
+newline
+a
+b/            1716884876  1000  1000  100644  8         `
+newline
+dir/file/       1716992150  1000  1000  100644  14        `
+file-in-a-dir

tests/test_util_ar.py

@@ -0,0 +1,86 @@
+import os
+import shutil
+import tempfile
+import unittest
+
+from osc.util.ar import Ar
+from osc.util.ar import ArError
+
+
+FIXTURES_DIR = os.path.join(os.path.dirname(__file__), "fixtures")
+
+
+class TestAr(unittest.TestCase):
+    def setUp(self):
+        self.tmpdir = tempfile.mkdtemp(prefix="osc_test_")
+        try:
+            self.old_cwd = os.getcwd()
+        except FileNotFoundError:
+            self.old_cwd = os.path.expanduser("~")
+        os.chdir(self.tmpdir)
+        self.archive = os.path.join(FIXTURES_DIR, "archive.ar")
+        self.ar = Ar(self.archive)
+        self.ar.read()
+
+    def tearDown(self):
+        os.chdir(self.old_cwd)
+        shutil.rmtree(self.tmpdir)
+
+    def test_file_list(self):
+        actual = [i.name for i in self.ar]
+        expected = [
+            # absolute path
+            b"/tmp/foo",
+            # this is a filename, not a long filename reference
+            b"/123",
+            b"very-long-long-long-long-name",
+            b"very-long-long-long-long-name2",
+            # long file name with a newline
+            b"very-long-name\n-with-newline",
+            # short file name with a newline
+            b"a\nb",
+            b"dir/file",
+        ]
+        self.assertEqual(actual, expected)
+
+    def test_get_file(self):
+        f = self.ar.get_file(b"/tmp/foo")
+        self.assertIsNotNone(f)
+
+        f = self.ar.get_file("/tmp/foo")
+        self.assertIsNotNone(f)
+
+        f = self.ar.get_file("does-not-exist")
+        self.assertIsNone(f)
+
+    def test_saveTo(self):
+        f = self.ar.get_file("a\nb")
+        path = f.saveTo(self.tmpdir)
+
+        # check that we've got the expected path
+        self.assertEqual(path, os.path.join(self.tmpdir, "a\nb"))
+
+        # ... and that the contents also match
+        with open(path, "r", encoding="utf-8") as f:
+            self.assertEqual(f.read(), "newline\n")
+
+    def test_saveTo_subdir(self):
+        f = self.ar.get_file("dir/file")
+        path = f.saveTo(self.tmpdir)
+
+        # check that we've got the expected path
+        self.assertEqual(path, os.path.join(self.tmpdir, "dir/file"))
+
+        # ... and that the contents also match
+        with open(path, "r", encoding="utf-8") as f:
+            self.assertEqual(f.read(), "file-in-a-dir\n")
+
+    def test_saveTo_abspath(self):
+        f = self.ar.get_file("/tmp/foo")
+        assert f is not None
+        # this is supposed to throw an error, extracting files with absolute paths might overwrite system files
+        self.assertRaises(ArError, f.saveTo, self.tmpdir)
+
+
+if __name__ == "__main__":
+    unittest.main()