gh-121285: Remove backtracking when parsing tarfile headers (GH-121286)

* Remove backtracking when parsing tarfile headers * Rewrite PAX header parsing to be stricter * Optimize parsing of GNU extended sparse headers v0.0 Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru> Co-authored-by: Gregory P. Smith <greg@krypto.org>

Author: 3 people

Lib/tarfile.py

@@ -845,6 +845,9 @@ def data_filter(member, dest_path):
 # Sentinel for replace() defaults, meaning "don't change the attribute"
 _KEEP = object()
 
+# Header length is digits followed by a space.
+_header_length_prefix_re = re.compile(br"([0-9]{1,20}) ")
+
 class TarInfo(object):
  """Informational class which holds the details about an
  archive member given by a tar header block.
@@ -1432,55 +1435,76 @@ def _proc_pax(self, tarfile):
  else:
  pax_headers = tarfile.pax_headers.copy()
 
- # Check if the pax header contains a hdrcharset field. This tells us
- # the encoding of the path, linkpath, uname and gname fields. Normally,
- # these fields are UTF-8 encoded but since POSIX.1-2008 tar
- # implementations are allowed to store them as raw binary strings if
- # the translation to UTF-8 fails.
- match = re.search(br"\d+ hdrcharset=([^\n]+)\n", buf)
- if match is not None:
- pax_headers["hdrcharset"] = match.group(1).decode("utf-8")
-
- # For the time being, we don't care about anything other than "BINARY".
- # The only other value that is currently allowed by the standard is
- # "ISO-IR 10646 2000 UTF-8" in other words UTF-8.
- hdrcharset = pax_headers.get("hdrcharset")
- if hdrcharset == "BINARY":
- encoding = tarfile.encoding
- else:
- encoding = "utf-8"
-
  # Parse pax header information. A record looks like that:
  # "%d %s=%s\n" % (length, keyword, value). length is the size
  # of the complete record including the length field itself and
- # the newline. keyword and value are both UTF-8 encoded strings.
- regex = re.compile(br"(\d+) ([^=]+)=")
+ # the newline.
  pos = 0
- while match := regex.match(buf, pos):
- length, keyword = match.groups()
- length = int(length)
- if length == 0:
+ encoding = None
+ raw_headers = []
+ while len(buf) > pos and buf[pos] != 0x00:
+ if not (match := _header_length_prefix_re.match(buf, pos)):
+ raise InvalidHeaderError("invalid header")
+ try:
+ length = int(match.group(1))
+ except ValueError:
+ raise InvalidHeaderError("invalid header")
+ # Headers must be at least 5 bytes, shortest being '5 x=\n'.
+ # Value is allowed to be empty.
+ if length < 5:
+ raise InvalidHeaderError("invalid header")
+ if pos + length > len(buf):
+ raise InvalidHeaderError("invalid header")
+
+ header_value_end_offset = match.start(1) + length - 1 # Last byte of the header
+ keyword_and_value = buf[match.end(1) + 1:header_value_end_offset]
+ raw_keyword, equals, raw_value = keyword_and_value.partition(b"=")
+
+ # Check the framing of the header. The last character must be '\n' (0x0A)
+ if not raw_keyword or equals != b"=" or buf[header_value_end_offset] != 0x0A:
  raise InvalidHeaderError("invalid header")
- value = buf[match.end(2) + 1:match.start(1) + length - 1]
+ raw_headers.append((length, raw_keyword, raw_value))
+
+ # Check if the pax header contains a hdrcharset field. This tells us
+ # the encoding of the path, linkpath, uname and gname fields. Normally,
+ # these fields are UTF-8 encoded but since POSIX.1-2008 tar
+ # implementations are allowed to store them as raw binary strings if
+ # the translation to UTF-8 fails. For the time being, we don't care about
+ # anything other than "BINARY". The only other value that is currently
+ # allowed by the standard is "ISO-IR 10646 2000 UTF-8" in other words UTF-8.
+ # Note that we only follow the initial 'hdrcharset' setting to preserve
+ # the initial behavior of the 'tarfile' module.
+ if raw_keyword == b"hdrcharset" and encoding is None:
+ if raw_value == b"BINARY":
+ encoding = tarfile.encoding
+ else: # This branch ensures only the first 'hdrcharset' header is used.
+ encoding = "utf-8"
 
+ pos += length
+
+ # If no explicit hdrcharset is set, we use UTF-8 as a default.
+ if encoding is None:
+ encoding = "utf-8"
+
+ # After parsing the raw headers we can decode them to text.
+ for length, raw_keyword, raw_value in raw_headers:
  # Normally, we could just use "utf-8" as the encoding and "strict"
  # as the error handler, but we better not take the risk. For
  # example, GNU tar <= 1.23 is known to store filenames it cannot
  # translate to UTF-8 as raw strings (unfortunately without a
  # hdrcharset=BINARY header).
  # We first try the strict standard encoding, and if that fails we
  # fall back on the user's encoding and error handler.
- keyword = self._decode_pax_field(keyword, "utf-8", "utf-8",
+ keyword = self._decode_pax_field(raw_keyword, "utf-8", "utf-8",
  tarfile.errors)
  if keyword in PAX_NAME_FIELDS:
- value = self._decode_pax_field(value, encoding, tarfile.encoding,
+ value = self._decode_pax_field(raw_value, encoding, tarfile.encoding,
  tarfile.errors)
  else:
- value = self._decode_pax_field(value, "utf-8", "utf-8",
+ value = self._decode_pax_field(raw_value, "utf-8", "utf-8",
  tarfile.errors)
 
  pax_headers[keyword] = value
- pos += length
 
  # Fetch the next header.
  try:
@@ -1495,7 +1519,7 @@ def _proc_pax(self, tarfile):
 
  elif "GNU.sparse.size" in pax_headers:
  # GNU extended sparse format version 0.0.
- self._proc_gnusparse_00(next, pax_headers, buf)
+ self._proc_gnusparse_00(next, raw_headers)
 
  elif pax_headers.get("GNU.sparse.major") == "1" and pax_headers.get("GNU.sparse.minor") == "0":
  # GNU extended sparse format version 1.0.
@@ -1517,15 +1541,24 @@ def _proc_pax(self, tarfile):
 
  return next
 
- def _proc_gnusparse_00(self, next, pax_headers, buf):
+ def _proc_gnusparse_00(self, next, raw_headers):
  """Process a GNU tar extended sparse header, version 0.0.
  """
  offsets = []
- for match in re.finditer(br"\d+ GNU.sparse.offset=(\d+)\n", buf):
- offsets.append(int(match.group(1)))
  numbytes = []
- for match in re.finditer(br"\d+ GNU.sparse.numbytes=(\d+)\n", buf):
- numbytes.append(int(match.group(1)))
+ for _, keyword, value in raw_headers:
+ if keyword == b"GNU.sparse.offset":
+ try:
+ offsets.append(int(value.decode()))
+ except ValueError:
+ raise InvalidHeaderError("invalid header")
+
+ elif keyword == b"GNU.sparse.numbytes":
+ try:
+ numbytes.append(int(value.decode()))
+ except ValueError:
+ raise InvalidHeaderError("invalid header")
+
  next.sparse = list(zip(offsets, numbytes))
 
  def _proc_gnusparse_01(self, next, pax_headers):

Lib/test/test_tarfile.py

@@ -1261,6 +1261,48 @@ def test_pax_number_fields(self):
  finally:
  tar.close()
 
+ def test_pax_header_bad_formats(self):
+ # The fields from the pax header have priority over the
+ # TarInfo.
+ pax_header_replacements = (
+ b" foo=bar\n",
+ b"0 \n",
+ b"1 \n",
+ b"2 \n",
+ b"3 =\n",
+ b"4 =a\n",
+ b"1000000 foo=bar\n",
+ b"0 foo=bar\n",
+ b"-12 foo=bar\n",
+ b"000000000000000000000000036 foo=bar\n",
+ )
+ pax_headers = {"foo": "bar"}
+
+ for replacement in pax_header_replacements:
+ with self.subTest(header=replacement):
+ tar = tarfile.open(tmpname, "w", format=tarfile.PAX_FORMAT,
+ encoding="iso8859-1")
+ try:
+ t = tarfile.TarInfo()
+ t.name = "pax" # non-ASCII
+ t.uid = 1
+ t.pax_headers = pax_headers
+ tar.addfile(t)
+ finally:
+ tar.close()
+
+ with open(tmpname, "rb") as f:
+ data = f.read()
+ self.assertIn(b"11 foo=bar\n", data)
+ data = data.replace(b"11 foo=bar\n", replacement)
+
+ with open(tmpname, "wb") as f:
+ f.truncate()
+ f.write(data)
+
+ with self.assertRaisesRegex(tarfile.ReadError, r"method tar: ReadError\('invalid header'\)"):
+ tarfile.open(tmpname, encoding="iso8859-1")
+
 
 class WriteTestBase(TarTest):
  # Put all write tests in here that are supposed to be tested

Misc/NEWS.d/next/Security/2024-07-02-13-39-20.gh-issue-121285.hrl-yI.rst

@@ -0,0 +1,2 @@
+Remove backtracking from tarfile header parsing for ``hdrcharset``, PAX, and
+GNU sparse headers.