selftests: net: Test UDP length overflow with PMTU discover and big MTU

Two previous commits fixed overflow of UDP length when setsockopt
IP(V6)_MTU_DISCOVER is set to IPV6_PMTUDISC_DO or IP(V6)_PMTUDISC_PROBE,
and a large packet is sent over a netdev with an unusually large MTU.

This commit adds the selftests that replicate the described steps to
reproduce for IPv6 and IPv4, and also one more test that ensures that
sending UDP jumbograms over a raw socket is still possible after the
fix.

Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260901195714.673548-4-alice.kernel@fastmail.im
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/Makefile

@@ -25,6 +25,7 @@ TEST_PROGS := \
 	cmsg_so_mark.sh \
 	cmsg_so_priority.sh \
 	cmsg_time.sh \
+	cork_fragsize.py \
 	double_udp_encap.sh \
 	drop_monitor_tests.sh \
 	ecmp_rehash.sh \

tools/testing/selftests/net/cork_fragsize.py

@@ -0,0 +1,187 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+'''Test possible UDP length overflow in udp_send_skb/udp_v6_send_skb.'''
+
+import errno
+import gzip
+import os
+import socket
+import struct
+import subprocess
+from contextlib import contextmanager
+
+from lib.py import (
+    KsftNamedVariant,
+    KsftSkipEx,
+    NetNS,
+    NetNSEnter,
+    defer,
+    ip,
+    ksft_eq,
+    ksft_exit,
+    ksft_pr,
+    ksft_raises,
+    ksft_run,
+    ksft_true,
+    ksft_variants,
+)
+
+IP_MTU_DISCOVER = 10
+IP_PMTUDISC_PROBE = 3
+IPV6_MTU_DISCOVER = 23
+IPV6_PMTUDISC_DO = 2
+IPV6_PMTUDISC_PROBE = 3
+IPV6_TLV_JUMBO = 194
+
+
+def check_kernel_config(option: str) -> bool | None:
+    '''
+    Check whether the option is enabled in the config of the running kernel.
+    Returns None if the config is not found; otherwise returns True/False
+    depending on the option value in the config.
+    '''
+
+    for filename, method in [
+        ('/proc/config.gz', gzip.open),
+        (f'/boot/config-{os.uname().release}', open),
+    ]:
+        try:
+            with method(filename, 'rt') as config:
+                for line in config:
+                    if line.rstrip() == f'{option}=y':
+                        return True
+                return False
+        except OSError:
+            continue
+        return None
+
+
+def assert_debug_kernel() -> None:
+    '''
+    Skip the test if CONFIG_DEBUG_NET is not set in the kernel config.
+    '''
+
+    res = check_kernel_config('CONFIG_DEBUG_NET')
+    if res is None:
+        ksft_pr("WARN: Can't read kernel config; assuming debug kernel, and running the test")
+    elif not res:
+        raise KsftSkipEx('CONFIG_DEBUG_NET is not set')
+
+
+def check_dmesg_clean(func: str) -> bool:
+    '''
+    Check if the given function produced a WARN in dmesg.
+    '''
+
+    with subprocess.Popen(['dmesg'], stdout=subprocess.PIPE) as dmesg:
+        res = subprocess.run(['grep', '-q', f'WARNING:.*{func}'], stdin=dmesg.stdout, check=False)
+    return res.returncode != 0 and dmesg.returncode == 0
+
+
+@contextmanager
+def dummy_netdev(ns: NetNS, mtu: int, ipv6: bool) -> None:
+    '''
+    Create a dummy netdev inside the given namespace, and tune it for the test.
+    '''
+
+    ip('link add dummy type dummy', ns=ns)
+    with defer(ip, 'link del dummy', ns=ns):
+        ip(f'link set dummy mtu {mtu}', ns=ns)
+        ip('link set dummy up', ns=ns)
+        flag = '-6' if ipv6 else ''
+        nodad = 'nodad' if ipv6 else ''
+        local = 'fd00::1/64' if ipv6 else '10.0.0.1/24'
+        remote = 'fd00::2' if ipv6 else '10.0.0.2'
+        ip(f'{flag} addr add {local} dev dummy {nodad}', ns=ns)
+        ip(f'{flag} neigh add {remote} lladdr 02:00:00:00:00:02 dev dummy nud permanent', ns=ns)
+        yield
+
+
+@ksft_variants([
+    KsftNamedVariant(
+        'ipv6',
+        True,
+        socket.AF_INET6,
+        (socket.IPPROTO_IPV6, IPV6_MTU_DISCOVER, IPV6_PMTUDISC_DO),
+        'fd00::2',
+        'udp_v6_send_skb',
+    ),
+    KsftNamedVariant(
+        'ipv4',
+        False,
+        socket.AF_INET,
+        (socket.IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_PROBE),
+        '10.0.0.2',
+        'udp_send_skb',
+    ),
+])
+def test_udp(
+    ipv6: bool,
+    af: socket.AddressFamily,
+    sockopts: tuple[int, int, int],
+    destip: str,
+    func: str
+) -> None:
+    '''
+    Test that sending an oversized UDP packet over a UDP socket doesn't overflow
+    the 16-bit length field in the UDP header, which could happen on older
+    kernels in udp_send_skb/udp_v6_send_skb.
+
+    IPv4: The packet will be dropped with EMSGSIZE, but the overflow could
+    happen before it happens. The only way to test this is to check dmesg on
+    CONFIG_DEBUG_NET=y kernels that have udp_set_len_short with the warning.
+
+    IPv6: The packet will be dropped with EMSGSIZE on fixed kernels, and will be
+    sent corrupted on older kernels. Test both: sendto must return EMSGSIZE, and
+    dmesg must be clean of warnings on CONFIG_DEBUG_NET=y kernels.
+    '''
+
+    if not ipv6:
+        assert_debug_kernel()
+
+    with (
+        NetNS() as ns,
+        dummy_netdev(ns, 65556 + 20 * ipv6, ipv6),
+        NetNSEnter(ns),
+        socket.socket(af, socket.SOCK_DGRAM) as fd,
+    ):
+        fd.setsockopt(*sockopts)
+        with ksft_raises(OSError) as e:
+            fd.sendto(b' ' * 65528, (destip, 1234))
+        # IPv6: EMSGSIZE happens on kernels with the fix.
+        # IPv4: EMSGSIZE happens on both fixed and unfixed kernels, after the
+        #       WARN is printed - ignore it and rely on the dmesg check.
+        if e.exception is not None:
+            ksft_eq(e.exception.errno, errno.EMSGSIZE)
+
+    ksft_true(check_dmesg_clean(func), 'WARNING detected in dmesg')
+
+
+def test_ipv6_jumbo() -> None:
+    '''
+    Test that sending UDP jumbograms over a raw IPv6 socket works, despite
+    having the fix for oversized UDP packets. sendto must not raise an OSError
+    exception (when raised, the test fails automatically).
+    '''
+
+    with (
+        NetNS() as ns,
+        dummy_netdev(ns, 65584, True),
+        NetNSEnter(ns),
+        socket.socket(socket.AF_INET6, socket.SOCK_RAW, socket.IPPROTO_UDP) as fd,
+    ):
+        hopopts = struct.pack('!BBBBI', 0, 0, IPV6_TLV_JUMBO, 4, 65544)
+        fd.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_HOPOPTS, hopopts)
+        fd.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_CHECKSUM, 6)
+        fd.setsockopt(socket.IPPROTO_IPV6, IPV6_MTU_DISCOVER, IPV6_PMTUDISC_PROBE)
+        udp = struct.pack('!HHHH', 1234, 1234, 0, 0) + b' ' * 65528
+        fd.sendto(udp, ('fd00::2', 0))
+
+
+if __name__ == "__main__":
+    ksft_run([
+        test_udp,
+        test_ipv6_jumbo,
+    ])
+    ksft_exit()