Issue #18805: better netmask validation in ipaddress

3 files changed, 139 insertions, 85 deletions

diff --git a/Lib/ipaddress.py b/Lib/ipaddress.py
index eabfd6f..ecf3f44 100644
--- a/Lib/ipaddress.py
+++ b/Lib/ipaddress.py
@@ -456,8 +456,8 @@ class _IPAddressBase(_TotalOrderingMixin):
             raise AddressValueError(msg % (address, address_len,
                                            expected_len, self._version))
 
-    def _ip_int_from_prefix(self, prefixlen=None):
-        """Turn the prefix length netmask into a int for comparison.
+    def _ip_int_from_prefix(self, prefixlen):
+        """Turn the prefix length into a bitwise netmask
 
         Args:
             prefixlen: An integer, the prefix length.
@@ -466,36 +466,92 @@ class _IPAddressBase(_TotalOrderingMixin):
             An integer.
 
         """
-        if prefixlen is None:
-            prefixlen = self._prefixlen
         return self._ALL_ONES ^ (self._ALL_ONES >> prefixlen)
 
-    def _prefix_from_ip_int(self, ip_int, mask=32):
-        """Return prefix length from the decimal netmask.
+    def _prefix_from_ip_int(self, ip_int):
+        """Return prefix length from the bitwise netmask.
 
         Args:
-            ip_int: An integer, the IP address.
-            mask: The netmask.  Defaults to 32.
+            ip_int: An integer, the netmask in axpanded bitwise format
+
+        Returns:
+            An integer, the prefix length.
+
+        Raises:
+            ValueError: If the input intermingles zeroes & ones
+        """
+        trailing_zeroes = _count_righthand_zero_bits(ip_int,
+                                                     self._max_prefixlen)
+        prefixlen = self._max_prefixlen - trailing_zeroes
+        leading_ones = ip_int >> trailing_zeroes
+        all_ones = (1 << prefixlen) - 1
+        if leading_ones != all_ones:
+            byteslen = self._max_prefixlen // 8
+            details = ip_int.to_bytes(byteslen, 'big')
+            msg = 'Netmask pattern %r mixes zeroes & ones'
+            raise ValueError(msg % details)
+        return prefixlen
+
+    def _report_invalid_netmask(self, netmask_str):
+        msg = '%r is not a valid netmask' % netmask_str
+        raise NetmaskValueError(msg) from None
+
+    def _prefix_from_prefix_string(self, prefixlen_str):
+        """Return prefix length from a numeric string
+
+        Args:
+            prefixlen_str: The string to be converted
 
         Returns:
             An integer, the prefix length.
 
+        Raises:
+            NetmaskValueError: If the input is not a valid netmask
         """
-        return mask - _count_righthand_zero_bits(ip_int, mask)
+        # int allows a leading +/- as well as surrounding whitespace,
+        # so we ensure that isn't the case
+        if not _BaseV4._DECIMAL_DIGITS.issuperset(prefixlen_str):
+            self._report_invalid_netmask(prefixlen_str)
+        try:
+            prefixlen = int(prefixlen_str)
+        except ValueError:
+            self._report_invalid_netmask(prefixlen_str)
+        if not (0 <= prefixlen <= self._max_prefixlen):
+            self._report_invalid_netmask(prefixlen_str)
+        return prefixlen
 
-    def _ip_string_from_prefix(self, prefixlen=None):
-        """Turn a prefix length into a dotted decimal string.
+    def _prefix_from_ip_string(self, ip_str):
+        """Turn a netmask/hostmask string into a prefix length
 
         Args:
-            prefixlen: An integer, the netmask prefix length.
+            ip_str: The netmask/hostmask to be converted
 
         Returns:
-            A string, the dotted decimal netmask string.
+            An integer, the prefix length.
 
+        Raises:
+            NetmaskValueError: If the input is not a valid netmask/hostmask
         """
-        if not prefixlen:
-            prefixlen = self._prefixlen
-        return self._string_from_ip_int(self._ip_int_from_prefix(prefixlen))
+        # Parse the netmask/hostmask like an IP address.
+        try:
+            ip_int = self._ip_int_from_string(ip_str)
+        except AddressValueError:
+            self._report_invalid_netmask(ip_str)
+
+        # Try matching a netmask (this would be /1*0*/ as a bitwise regexp).
+        # Note that the two ambiguous cases (all-ones and all-zeroes) are
+        # treated as netmasks.
+        try:
+            return self._prefix_from_ip_int(ip_int)
+        except ValueError:
+            pass
+
+        # Invert the bits, and try matching a /0+1+/ hostmask instead.
+        ip_int ^= self._ALL_ONES
+        try:
+            return self._prefix_from_ip_int(ip_int)
+        except ValueError:
+            self._report_invalid_netmask(ip_str)
 
 
 class _BaseAddress(_IPAddressBase):
@@ -504,7 +560,6 @@ class _BaseAddress(_IPAddressBase):
 
     This IP class contains the version independent methods which are
     used by single IP addresses.
-
     """
 
     def __init__(self, address):
@@ -873,7 +928,7 @@ class _BaseNetwork(_IPAddressBase):
             raise ValueError('prefix length diff must be > 0')
         new_prefixlen = self._prefixlen + prefixlen_diff
 
-        if not self._is_valid_netmask(str(new_prefixlen)):
+        if new_prefixlen > self._max_prefixlen:
             raise ValueError(
                 'prefix length diff %d is invalid for netblock %s' % (
                     new_prefixlen, self))
@@ -1428,33 +1483,16 @@ class IPv4Network(_BaseV4, _BaseNetwork):
         self.network_address = IPv4Address(self._ip_int_from_string(addr[0]))
 
         if len(addr) == 2:
-            mask = addr[1].split('.')
-
-            if len(mask) == 4:
-                # We have dotted decimal netmask.
-                if self._is_valid_netmask(addr[1]):
-                    self.netmask = IPv4Address(self._ip_int_from_string(
-                            addr[1]))
-                elif self._is_hostmask(addr[1]):
-                    self.netmask = IPv4Address(
-                        self._ip_int_from_string(addr[1]) ^ self._ALL_ONES)
-                else:
-                    raise NetmaskValueError('%r is not a valid netmask'
-                                                     % addr[1])
-
-                self._prefixlen = self._prefix_from_ip_int(int(self.netmask))
-            else:
-                # We have a netmask in prefix length form.
-                if not self._is_valid_netmask(addr[1]):
-                    raise NetmaskValueError('%r is not a valid netmask'
-                                                     % addr[1])
-                self._prefixlen = int(addr[1])
-                self.netmask = IPv4Address(self._ip_int_from_prefix(
-                    self._prefixlen))
+            try:
+                # Check for a netmask in prefix length form
+                self._prefixlen = self._prefix_from_prefix_string(addr[1])
+            except NetmaskValueError:
+                # Check for a netmask or hostmask in dotted-quad form.
+                # This may raise NetmaskValueError.
+                self._prefixlen = self._prefix_from_ip_string(addr[1])
         else:
             self._prefixlen = self._max_prefixlen
-            self.netmask = IPv4Address(self._ip_int_from_prefix(
-                self._prefixlen))
+        self.netmask = IPv4Address(self._ip_int_from_prefix(self._prefixlen))
 
         if strict:
             if (IPv4Address(int(self.network_address) & int(self.netmask)) !=
@@ -2042,11 +2080,8 @@ class IPv6Network(_BaseV6, _BaseNetwork):
         self.network_address = IPv6Address(self._ip_int_from_string(addr[0]))
 
         if len(addr) == 2:
-            if self._is_valid_netmask(addr[1]):
-                self._prefixlen = int(addr[1])
-            else:
-                raise NetmaskValueError('%r is not a valid netmask'
-                                                     % addr[1])
+            # This may raise NetmaskValueError
+            self._prefixlen = self._prefix_from_prefix_string(addr[1])
         else:
             self._prefixlen = self._max_prefixlen
 
@@ -2061,23 +2096,6 @@ class IPv6Network(_BaseV6, _BaseNetwork):
         if self._prefixlen == (self._max_prefixlen - 1):
             self.hosts = self.__iter__
 
-    def _is_valid_netmask(self, prefixlen):
-        """Verify that the netmask/prefixlen is valid.
-
-        Args:
-            prefixlen: A string, the netmask in prefix length format.
-
-        Returns:
-            A boolean, True if the prefix represents a valid IPv6
-            netmask.
-
-        """
-        try:
-            prefixlen = int(prefixlen)
-        except ValueError:
-            return False
-        return 0 <= prefixlen <= self._max_prefixlen
-
     @property
     def is_site_local(self):
         """Test if the address is reserved for site-local.
diff --git a/Lib/test/test_ipaddress.py b/Lib/test/test_ipaddress.py
index 99c54f1..5da7f45 100644
--- a/Lib/test/test_ipaddress.py
+++ b/Lib/test/test_ipaddress.py
@@ -398,18 +398,47 @@ class NetmaskTestMixin_v4(CommonTestMixin_v4):
         assertBadAddress("::1.2.3.4", "Only decimal digits")
         assertBadAddress("1.2.3.256", re.escape("256 (> 255)"))
 
+    def test_valid_netmask(self):
+        self.assertEqual(str(self.factory('192.0.2.0/255.255.255.0')),
+                         '192.0.2.0/24')
+        for i in range(0, 33):
+            # Generate and re-parse the CIDR format (trivial).
+            net_str = '0.0.0.0/%d' % i
+            net = self.factory(net_str)
+            self.assertEqual(str(net), net_str)
+            # Generate and re-parse the expanded netmask.
+            self.assertEqual(
+                str(self.factory('0.0.0.0/%s' % net.netmask)), net_str)
+            # Zero prefix is treated as decimal.
+            self.assertEqual(str(self.factory('0.0.0.0/0%d' % i)), net_str)
+            # Generate and re-parse the expanded hostmask.  The ambiguous
+            # cases (/0 and /32) are treated as netmasks.
+            if i in (32, 0):
+                net_str = '0.0.0.0/%d' % (32 - i)
+            self.assertEqual(
+                str(self.factory('0.0.0.0/%s' % net.hostmask)), net_str)
+
     def test_netmask_errors(self):
         def assertBadNetmask(addr, netmask):
-            msg = "%r is not a valid netmask"
-            with self.assertNetmaskError(msg % netmask):
+            msg = "%r is not a valid netmask" % netmask
+            with self.assertNetmaskError(re.escape(msg)):
                 self.factory("%s/%s" % (addr, netmask))
 
         assertBadNetmask("1.2.3.4", "")
+        assertBadNetmask("1.2.3.4", "-1")
+        assertBadNetmask("1.2.3.4", "+1")
+        assertBadNetmask("1.2.3.4", " 1 ")
+        assertBadNetmask("1.2.3.4", "0x1")
         assertBadNetmask("1.2.3.4", "33")
         assertBadNetmask("1.2.3.4", "254.254.255.256")
+        assertBadNetmask("1.2.3.4", "1.a.2.3")
         assertBadNetmask("1.1.1.1", "254.xyz.2.3")
         assertBadNetmask("1.1.1.1", "240.255.0.0")
+        assertBadNetmask("1.1.1.1", "255.254.128.0")
+        assertBadNetmask("1.1.1.1", "0.1.127.255")
         assertBadNetmask("1.1.1.1", "pudding")
+        assertBadNetmask("1.1.1.1", "::")
+
 
 class InterfaceTestCase_v4(BaseTestCase, NetmaskTestMixin_v4):
     factory = ipaddress.IPv4Interface
@@ -438,17 +467,34 @@ class NetmaskTestMixin_v6(CommonTestMixin_v6):
         assertBadAddress("10/8", "At least 3 parts")
         assertBadAddress("1234:axy::b", "Only hex digits")
 
+    def test_valid_netmask(self):
+        # We only support CIDR for IPv6, because expanded netmasks are not
+        # standard notation.
+        self.assertEqual(str(self.factory('2001:db8::/32')), '2001:db8::/32')
+        for i in range(0, 129):
+            # Generate and re-parse the CIDR format (trivial).
+            net_str = '::/%d' % i
+            self.assertEqual(str(self.factory(net_str)), net_str)
+            # Zero prefix is treated as decimal.
+            self.assertEqual(str(self.factory('::/0%d' % i)), net_str)
+
     def test_netmask_errors(self):
         def assertBadNetmask(addr, netmask):
-            msg = "%r is not a valid netmask"
-            with self.assertNetmaskError(msg % netmask):
+            msg = "%r is not a valid netmask" % netmask
+            with self.assertNetmaskError(re.escape(msg)):
                 self.factory("%s/%s" % (addr, netmask))
 
         assertBadNetmask("::1", "")
         assertBadNetmask("::1", "::1")
         assertBadNetmask("::1", "1::")
+        assertBadNetmask("::1", "-1")
+        assertBadNetmask("::1", "+1")
+        assertBadNetmask("::1", " 1 ")
+        assertBadNetmask("::1", "0x1")
         assertBadNetmask("::1", "129")
+        assertBadNetmask("::1", "1.2.3.4")
         assertBadNetmask("::1", "pudding")
+        assertBadNetmask("::", "::")
 
 class InterfaceTestCase_v6(BaseTestCase, NetmaskTestMixin_v6):
     factory = ipaddress.IPv6Interface
@@ -694,16 +740,14 @@ class IpaddrUnitTest(unittest.TestCase):
     def testZeroNetmask(self):
         ipv4_zero_netmask = ipaddress.IPv4Interface('1.2.3.4/0')
         self.assertEqual(int(ipv4_zero_netmask.network.netmask), 0)
-        self.assertTrue(ipv4_zero_netmask.network._is_valid_netmask(
-                str(0)))
+        self.assertEqual(ipv4_zero_netmask._prefix_from_prefix_string('0'), 0)
         self.assertTrue(ipv4_zero_netmask._is_valid_netmask('0'))
         self.assertTrue(ipv4_zero_netmask._is_valid_netmask('0.0.0.0'))
         self.assertFalse(ipv4_zero_netmask._is_valid_netmask('invalid'))
 
         ipv6_zero_netmask = ipaddress.IPv6Interface('::1/0')
         self.assertEqual(int(ipv6_zero_netmask.network.netmask), 0)
-        self.assertTrue(ipv6_zero_netmask.network._is_valid_netmask(
-                str(0)))
+        self.assertEqual(ipv6_zero_netmask._prefix_from_prefix_string('0'), 0)
 
     def testIPv4NetAndHostmasks(self):
         net = self.ipv4_network
@@ -719,7 +763,7 @@ class IpaddrUnitTest(unittest.TestCase):
         self.assertFalse(net._is_hostmask('1.2.3.4'))
 
         net = ipaddress.IPv4Network('127.0.0.0/0.0.0.255')
-        self.assertEqual(24, net.prefixlen)
+        self.assertEqual(net.prefixlen, 24)
 
     def testGetBroadcast(self):
         self.assertEqual(int(self.ipv4_network.broadcast_address), 16909311)
@@ -1271,11 +1315,6 @@ class IpaddrUnitTest(unittest.TestCase):
         self.assertEqual(ipaddress.IPv6Interface('::1:0:0:0:0').packed,
                          b'\x00' * 6 + b'\x00\x01' + b'\x00' * 8)
 
-    def testIpStrFromPrefixlen(self):
-        ipv4 = ipaddress.IPv4Interface('1.2.3.4/24')
-        self.assertEqual(ipv4._ip_string_from_prefix(), '255.255.255.0')
-        self.assertEqual(ipv4._ip_string_from_prefix(28), '255.255.255.240')
-
     def testIpType(self):
         ipv4net = ipaddress.ip_network('1.2.3.4')
         ipv4addr = ipaddress.ip_address('1.2.3.4')
@@ -1467,14 +1506,8 @@ class IpaddrUnitTest(unittest.TestCase):
     def testIPBases(self):
         net = self.ipv4_network
         self.assertEqual('1.2.3.0/24', net.compressed)
-        self.assertEqual(
-                net._ip_int_from_prefix(24),
-                net._ip_int_from_prefix(None))
         net = self.ipv6_network
         self.assertRaises(ValueError, net._string_from_ip_int, 2**128 + 1)
-        self.assertEqual(
-                self.ipv6_address._string_from_ip_int(self.ipv6_address._ip),
-                self.ipv6_address._string_from_ip_int(None))
 
     def testIPv6NetworkHelpers(self):
         net = self.ipv6_network
diff --git a/Misc/NEWS b/Misc/NEWS
index adce2c1..4df6c23 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -48,6 +48,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #18805: the netmask/hostmask parsing in ipaddress now more reliably
+  filters out illegal values
+
 - Issue #17369: get_filename was raising an exception if the filename
   parameter's RFC2231 encoding was broken in certain ways.  This was
   a regression relative to python2.