gh-119342: Fix OOM vulnerability in plistlib

Lib/plistlib.py

@@ -73,6 +73,9 @@
 PlistFormat = enum.Enum('PlistFormat', 'FMT_XML FMT_BINARY', module=__name__)
 globals().update(PlistFormat.__members__)
 
+# Data larger than this will be read in chunks, to prevent extreme
+# overallocation.
+_SAFE_BUF_SIZE = 1 << 20
 
 class UID:
     def __init__(self, data):
@@ -508,12 +511,24 @@ def _get_size(self, tokenL):
 
         return tokenL
 
+    def _read(self, size):
+        cursize = min(size, _SAFE_BUF_SIZE)
+        data = self._fp.read(cursize)
+        while True:
+            if len(data) != cursize:
+                raise InvalidFileException
+            if cursize == size:
+                return data
+            delta = min(cursize, size - cursize)
+            data += self._fp.read(delta)
+            cursize += delta
+
     def _read_ints(self, n, size):
-        data = self._fp.read(size * n)
+        data = self._read(size * n)
         if size in _BINARY_FORMAT:
             return struct.unpack(f'>{n}{_BINARY_FORMAT[size]}', data)
         else:
-            if not size or len(data) != size * n:
+            if not size:
                 raise InvalidFileException()
             return tuple(int.from_bytes(data[i: i + size], 'big')
                          for i in range(0, size * n, size))
@@ -573,22 +588,16 @@ def _read_object(self, ref):
 
         elif tokenH == 0x40:  # data
             s = self._get_size(tokenL)
-            result = self._fp.read(s)
-            if len(result) != s:
-                raise InvalidFileException()
+            result = self._read(s)
 
         elif tokenH == 0x50:  # ascii string
             s = self._get_size(tokenL)
-            data = self._fp.read(s)
-            if len(data) != s:
-                raise InvalidFileException()
+            data = self._read(s)
             result = data.decode('ascii')
 
         elif tokenH == 0x60:  # unicode string
             s = self._get_size(tokenL) * 2
-            data = self._fp.read(s)
-            if len(data) != s:
-                raise InvalidFileException()
+            data = self._read(s)
             result = data.decode('utf-16be')
 
         elif tokenH == 0x80:  # UID

Lib/test/test_plistlib.py

@@ -904,8 +904,7 @@ def test_dump_naive_datetime_with_aware_datetime_option(self):
 
 class TestBinaryPlistlib(unittest.TestCase):
 
-    @staticmethod
-    def decode(*objects, offset_size=1, ref_size=1):
+    def build(self, *objects, offset_size=1, ref_size=1):
         data = [b'bplist00']
         offset = 8
         offsets = []
@@ -917,7 +916,11 @@ def decode(*objects, offset_size=1, ref_size=1):
                            len(objects), 0, offset)
         data.extend(offsets)
         data.append(tail)
-        return plistlib.loads(b''.join(data), fmt=plistlib.FMT_BINARY)
+        return b''.join(data)
+
+    def decode(self, *objects, offset_size=1, ref_size=1):
+        data = self.build(*objects, offset_size=offset_size, ref_size=ref_size)
+        return plistlib.loads(data, fmt=plistlib.FMT_BINARY)
 
     def test_nonstandard_refs_size(self):
         # Issue #21538: Refs and offsets are 24-bit integers
@@ -1025,6 +1028,34 @@ def test_invalid_binary(self):
                 with self.assertRaises(plistlib.InvalidFileException):
                     plistlib.loads(b'bplist00' + data, fmt=plistlib.FMT_BINARY)
 
+    def test_truncated_large_data(self):
+        self.addCleanup(os_helper.unlink, os_helper.TESTFN)
+        def check(data):
+            with open(os_helper.TESTFN, 'wb') as f:
+                f.write(data)
+            # buffered file
+            with open(os_helper.TESTFN, 'rb') as f:
+                with self.assertRaises(plistlib.InvalidFileException):
+                    plistlib.load(f, fmt=plistlib.FMT_BINARY)
+            # unbuffered file
+            with open(os_helper.TESTFN, 'rb', buffering=0) as f:
+                with self.assertRaises(plistlib.InvalidFileException):
+                    plistlib.load(f, fmt=plistlib.FMT_BINARY)
+        for w in range(20, 64):
+            s = 1 << w
+            # data
+            check(self.build(b'\x4f\x13' + s.to_bytes(8, 'big')))
+            # ascii string
+            check(self.build(b'\x5f\x13' + s.to_bytes(8, 'big')))
+            # unicode string
+            check(self.build(b'\x6f\x13' + s.to_bytes(8, 'big')))
+            # array
+            check(self.build(b'\xaf\x13' + s.to_bytes(8, 'big')))
+            # dict
+            check(self.build(b'\xdf\x13' + s.to_bytes(8, 'big')))
+            # number of objects
+            check(b'bplist00' + struct.pack('>6xBBQQQ', 1, 1, s, 0, 8))
+
     def test_load_aware_datetime(self):
         data = (b'bplist003B\x04>\xd0d\x00\x00\x00\x08\x00\x00\x00\x00\x00\x00'
                 b'\x01\x01\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00'

Misc/NEWS.d/next/Security/2024-05-21-22-11-31.gh-issue-119342.BTFj4Z.rst

@@ -0,0 +1,2 @@
+Fix OOM vulnerability in :mod:`plistlib`, when reading a specially prepared
+small Plist file could cause consuming an arbitrary amount of memory.