[3.10] gh-119342: Fix a potential denial of service in plistlib (GH-119343) (#142151)

Reading a specially prepared small Plist file could cause OOM because file's
read(n) preallocates a bytes object for reading the specified amount of
data. Now plistlib reads large data by chunks, therefore the upper limit of
consumed memory is proportional to the size of the input file.
(cherry picked from commit 694922cf40aa3a28f898b5f5ee08b71b4922df70)

diff --git a/Lib/plistlib.py b/Lib/plistlib.py
index d6c997efe9c5f51b4bf5f0f43656e0ccca977f58..c80dfee02a33358f643bdad2aab83c0ff4b24678 100644 (file)
--- a/Lib/plistlib.py
+++ b/Lib/plistlib.py
@@ -73,6 +73,9 @@ from xml.parsers.expat import ParserCreate
 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.
+_MIN_READ_BUF_SIZE = 1 << 20
 
 class UID:
     def __init__(self, data):
@@ -499,12 +502,24 @@ class _BinaryPlistParser:
 
         return tokenL
 
+    def _read(self, size):
+        cursize = min(size, _MIN_READ_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))
@@ -561,22 +576,16 @@ class _BinaryPlistParser:
 
         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

diff --git a/Lib/test/test_plistlib.py b/Lib/test/test_plistlib.py
index ef96c6ceda21a28de20ca698bcf8b0225324567f..d3836991d212cde599fd08ac825f959a9d40cbd1 100644 (file)
--- a/Lib/test/test_plistlib.py
+++ b/Lib/test/test_plistlib.py
@@ -838,8 +838,7 @@ class TestPlistlib(unittest.TestCase):
 
 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 = []
@@ -851,7 +850,11 @@ class TestBinaryPlistlib(unittest.TestCase):
                            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
@@ -959,6 +962,34 @@ class TestBinaryPlistlib(unittest.TestCase):
                 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))
+
 
 class TestKeyedArchive(unittest.TestCase):
     def test_keyed_archive_data(self):

diff --git a/Misc/NEWS.d/next/Security/2024-05-21-22-11-31.gh-issue-119342.BTFj4Z.rst b/Misc/NEWS.d/next/Security/2024-05-21-22-11-31.gh-issue-119342.BTFj4Z.rst
new file mode 100644 (file)
index 0000000..04fd8fa
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2024-05-21-22-11-31.gh-issue-119342.BTFj4Z.rst
@@ -0,0 +1,5 @@
+Fix a potential memory denial of service in the :mod:`plistlib` module.
+When reading a Plist file received from untrusted source, it could cause
+an arbitrary amount of memory to be allocated.
+This could have led to symptoms including a :exc:`MemoryError`, swapping, out
+of memory (OOM) killed processes or containers, or even system crashes.