Re: [PATCH net v2 1/2] net: Fix UDP length overflow with PMTU discover and big MTU
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: 2026-08-13 20:11:02
Also in:
linux-kselftest
Alice Mikityanska wrote:
From: Alice Mikityanska <redacted>
This commit bounds cork->base.fragsize to IP(6)_MAX_MTU to avoid a
possible overflow of UDP length that triggers a WARN in
udp_set_len_short 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.
Steps to reproduce (included in the new selftest):
1. Set device MTU bigger than IP6_MAX_MTU (or IP_MAX_MTU + 20).
cork->base.fragsize will be set to that MTU in ip(6)_setup_cork.
2. Set IP(V6)_MTU_DISCOVER to IP(V6)_PMTUDISC_PROBE or IPV6_PMTUDISC_DO.
It lets maxnonfragsize be set to device MTU (cork->fragsize) in
__ip(6)_append_data, rather than to IP(6)_MAX_MTU.
3. Send 65528 bytes of payload (+8 bytes of UDP header, +20/40 bytes of
IPv4/IPv6 header). Device MTU allows it (it's only one byte bigger
than IP6_MAX_MTU or IP_MAX_MTU + IPv4 header, and the device MTU is
bigger than that).
4. The UDP length in the built packet is 65536, which overflows the
16-bit length field and triggers the WARN in udp_set_len_short.
Note: IP_PMTUDISC_DO with IPv4 is safe, because ip_dst_mtu_maybe_forward
always clamps at IP_MAX_MTU, unlike ip6_dst_mtu_maybe_forward.
The Fixes tag points at the first commit where I could reproduce the
overflow with IPv4 and IP_PMTUDISC_PROBE.
Fixes: daba287b299e ("ipv4: fix DO and PROBE pmtu mode regarding local fragmentation with UFO/CORK")
Reported-by: syzbot+ce13c07d96d04716eaa2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a6a966c.86abc875.e5c3d.0054.GAE@google.com/ (local)
Signed-off-by: Alice Mikityanska <redacted>
Assisted-by: Claude:claude-sonnet-4.6
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>Not sure whether IPv4 and IPv6 should be separate patches. IPv6 will have a different Fixes tag. Per maintainer-netdev.rst, mixing code and test changes in the same patch is also discouraged.
quoted hunk ↗ jump to hunk
--- net/ipv4/ip_output.c | 2 + net/ipv6/ip6_output.c | 2 +- tools/testing/selftests/net/Makefile | 1 + tools/testing/selftests/net/cork_fragsize.py | 77 ++++++++++++++++++++ 4 files changed, 81 insertions(+), 1 deletion(-) create mode 100755 tools/testing/selftests/net/cork_fragsize.pydiff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c index e6dd1e5b8c32..2bc997c4fa7b 100644 --- a/net/ipv4/ip_output.c +++ b/net/ipv4/ip_output.c@@ -1299,6 +1299,8 @@ static int ip_setup_cork(struct sock *sk, struct inet_cork *cork, cork->fragsize = ip_sk_use_pmtu(sk) ? dst4_mtu(&rt->dst) : READ_ONCE(rt->dst.dev->mtu); + if (cork->fragsize > IP_MAX_MTU) + cork->fragsize = IP_MAX_MTU;
if respinning: use min(), like IPv6? It's even more concise and easier to read.
quoted hunk ↗ jump to hunk
if (!inetdev_valid_mtu(cork->fragsize)) return -ENETUNREACH;diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c index 2c44e5ed6171..81fb61b94759 100644 --- a/net/ipv6/ip6_output.c +++ b/net/ipv6/ip6_output.c@@ -1430,7 +1430,7 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork, if (frag_size && frag_size < mtu) mtu = frag_size; - cork->base.fragsize = mtu; + cork->base.fragsize = min(mtu, IP6_MAX_MTU);
Not sure whether a limitation that we seriously have to care about, but: This restricts all IPv6 protocols, while only UDP has the length field limitation. With IPv6 jumbo frames and RAW sockets, could there be a valid case for larger fragsize?
quoted hunk ↗ jump to hunk
cork->base.gso_size = ipc6->gso_size; cork->base.tx_flags = 0; cork->base.mark = ipc6->sockc.mark;diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile index 708d960ae07d..82a1407f6a69 100644 --- a/tools/testing/selftests/net/Makefile +++ b/tools/testing/selftests/net/Makefile@@ -24,6 +24,7 @@ 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 \diff --git a/tools/testing/selftests/net/cork_fragsize.py b/tools/testing/selftests/net/cork_fragsize.py new file mode 100755 index 000000000000..7a1bb069227a --- /dev/null +++ b/tools/testing/selftests/net/cork_fragsize.py@@ -0,0 +1,77 @@ +#!/usr/bin/env python3 +# SPDX-License-Identifier: GPL-2.0 + +# Test possible UDP length overflow in udp_send_skb/udp_v6_send_skb. + +from lib.py import ksft_run, ksft_exit, ksft_true +from lib.py import ip, NetNS, NetNSEnter +import errno +import socket +import subprocess + + +IP_MTU_DISCOVER = 10 +IP_PMTUDISC_PROBE = 3 +IPV6_MTU_DISCOVER = 23 +IPV6_PMTUDISC_DO = 2 + + +def check_dmesg_clean(func) -> bool: + dmesg = subprocess.Popen(['dmesg'], stdout=subprocess.PIPE) + result = subprocess.run(['grep', '-q', f'WARNING:.*{func}'], stdin=dmesg.stdout) + dmesg.wait() + return result.returncode != 0 and dmesg.returncode == 0 + + +def test_ipv6() -> None: + with NetNS() as ns: + ip('link add dummy type dummy', ns=ns) + ip('link set dummy mtu 65576', ns=ns) + ip('link set dummy up', ns=ns) + ip('-6 addr add fd00::1/64 dev dummy nodad', ns=ns) + ip('-6 neigh add fd00::2 lladdr 02:00:00:00:00:02 dev dummy nud permanent', ns=ns) + + with NetNSEnter(ns): + with socket.socket(socket.AF_INET6, socket.SOCK_DGRAM) as fd: + fd.setsockopt(socket.IPPROTO_IPV6, IPV6_MTU_DISCOVER, IPV6_PMTUDISC_DO) + try: + fd.sendto(b' ' * 65528, ('fd00::2', 1234)) + except OSError as e: + # Ignore EMSGSIZE: it happens on kernels with the fix. + if e.errno != errno.EMSGSIZE: + raise + + ip('link del dummy', ns=ns) + + ksft_true(check_dmesg_clean('udp_v6_send_skb'), 'WARNING detected in dmesg') + + +def test_ipv4() -> None: + with NetNS() as ns: + ip('link add dummy type dummy', ns=ns) + ip('link set dummy mtu 65556', ns=ns) + ip('link set dummy up', ns=ns) + ip('addr add 10.0.0.1/24 dev dummy', ns=ns) + ip('neigh add 10.0.0.2 lladdr 02:00:00:00:00:02 dev dummy nud permanent', ns=ns) + + with NetNSEnter(ns): + with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as fd: + fd.setsockopt(socket.IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_PROBE) + try: + fd.sendto(b' ' * 65528, ('10.0.0.2', 1234)) + except OSError as e: + # Ignore EMSGSIZE: the check happens after the WARN is printed. + if e.errno != errno.EMSGSIZE: + raise + + ip('link del dummy', ns=ns) + + ksft_true(check_dmesg_clean('udp_send_skb'), 'WARNING detected in dmesg') + + +if __name__ == "__main__": + ksft_run([ + test_ipv6, + test_ipv4, + ]) + ksft_exit()-- 2.55.0