[PATCH net v4 0/4] Fix UDP length overflow in edge cases

[PATCH net v4 0/4] Fix UDP length overflow in edge cases

[Alice Mikityanska]

From: Alice Mikityanska <alice@isovalent.com>

These are fixes for rare edge cases of 16-bit UDP length field overflow
that might happen on netdevs with MTU >= 64k.

Exposed by the new WARN added to udp_set_len_short, reported by syzbot.

v4 changes: Fixed IPv6 commit message; deduplicated code in the selftest
and fixed pylint and ruff warnings.

v3: https://lore.kernel.org/netdev/20260822115717.1161782-1-alice.kernel@fastmail.im/

v3 changes: Split IPv4, IPv6 and selftest; limited the clamp to UDP
sockets to support UDP jumbograms over raw sockets; added a selftest for
raw sockets; added the kernel config check to the selftest.

v2: https://lore.kernel.org/netdev/20260813120351.2807829-1-alice.kernel@fastmail.im/

v2 changes: Restored the MTU clamp in ip6_dst_mtu_maybe_forward.

v1: https://lore.kernel.org/netdev/20260805205957.1652619-1-alice.kernel@fastmail.im/

Alice Mikityanska (4):
  net: ipv4: Fix UDP length overflow with PMTU discover and big MTU
  net: ipv6: Fix UDP length overflow with PMTU discover and big MTU
  selftests: net: Test UDP length overflow with PMTU discover and big
    MTU
  net: ipv6: Clamp to IP6_MAX_MTU in ip6_dst_mtu_maybe_forward

 include/net/ip6_route.h                      |   2 +
 net/ipv4/ip_output.c                         |   1 +
 net/ipv6/ip6_output.c                        |   2 +
 tools/testing/selftests/net/Makefile         |   1 +
 tools/testing/selftests/net/cork_fragsize.py | 140 +++++++++++++++++++
 5 files changed, 146 insertions(+)
 create mode 100755 tools/testing/selftests/net/cork_fragsize.py

-- 
2.55.0

[PATCH net v4 1/4] net: ipv4: Fix UDP length overflow with PMTU discover and big MTU

[Alice Mikityanska]

From: Alice Mikityanska <alice@isovalent.com>

This commit bounds cork->base.fragsize to IP_MAX_MTU to avoid a
possible overflow of UDP length that triggers a WARN in
udp_set_len_short when setsockopt IP_MTU_DISCOVER is set to
IP_PMTUDISC_PROBE, and a large packet is sent over a netdev with an
unusually large MTU.

Steps to reproduce:

1. Set device MTU bigger than IP_MAX_MTU + 20. cork->base.fragsize will
   be set to that MTU in ip_setup_cork.
2. Set IP_MTU_DISCOVER to IP_PMTUDISC_PROBE. It lets maxnonfragsize be
   set to device MTU (cork->fragsize) in __ip_append_data, rather than
   to IP_MAX_MTU.
3. Send 65528 bytes of payload (+8 bytes of UDP header, +20 bytes of
   IPv4 header). Device MTU allows it (it's only one byte bigger than
   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/
Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Assisted-by: Claude:claude-sonnet-4.6
Reviewed-by: Willem de Bruijn <willemb@google.com>
---
 net/ipv4/ip_output.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 74e095b6b7ca..a24cc8ee11d3 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1303,6 +1303,7 @@ 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);
+	cork->fragsize = min(cork->fragsize, IP_MAX_MTU);
 
 	if (!inetdev_valid_mtu(cork->fragsize))
 		return -ENETUNREACH;
-- 
2.55.0

[PATCH net v4 2/4] net: ipv6: Fix UDP length overflow with PMTU discover and big MTU

[Alice Mikityanska]

From: Alice Mikityanska <alice@isovalent.com>

This commit bounds cork->base.fragsize to IP6_MAX_MTU for UDP sockets to
avoid a possible overflow of UDP length that triggers a WARN in
udp_set_len_short when setsockopt IPV6_MTU_DISCOVER is set to
IPV6_PMTUDISC_DO or IPV6_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. cork->base.fragsize will be
   set to that MTU in ip6_setup_cork.
2. Set IPV6_MTU_DISCOVER to IPV6_PMTUDISC_PROBE or IPV6_PMTUDISC_DO. It
   lets maxnonfragsize be set to device MTU (cork->fragsize) in
   __ip6_append_data, rather than to IP6_MAX_MTU.
3. Send 65528 bytes of payload (+8 bytes of UDP header, +40 bytes of
   IPv6 header). Device MTU allows it (it's only one byte bigger than
   IP6_MAX_MTU, 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.

To avoid breaking sending UDP jumbograms over raw IPv6 sockets, limit
the change to UDP sockets only.

The original overflow bug with IPv6 and IPV6_PMTUDISC_DO seems to
predate git history (verified reproduction on 2.6.21), was fixed later,
and then reappeared in commit 427faee167bc ("net: ipv6: introduce
ip6_dst_mtu_maybe_forward"), which is chosen as the Fixes tag here. The
overflow with IPV6_PMTUDISC_PROBE reproduces since its introduction in
commit 628a5c561890 ("[INET]: Add IP(V6)_PMTUDISC_RPOBE").

Fixes: 427faee167bc ("net: ipv6: introduce ip6_dst_mtu_maybe_forward")
Reported-by: syzbot+ce13c07d96d04716eaa2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a6a966c.86abc875.e5c3d.0054.GAE@google.com/
Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Assisted-by: Codex:gpt-5.6-sol
Reviewed-by: Willem de Bruijn <willemb@google.com>
---
 net/ipv6/ip6_output.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 8fc4766c8da9..550965058991 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1432,6 +1432,8 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,
 	if (frag_size && frag_size < mtu)
 		mtu = frag_size;
 
+	if (sk_is_udp(sk))
+		mtu = min(mtu, IP6_MAX_MTU);
 	cork->base.fragsize = mtu;
 	cork->base.gso_size = ipc6->gso_size;
 	cork->base.tx_flags = 0;
-- 
2.55.0

[PATCH net v4 3/4] selftests: net: Test UDP length overflow with PMTU discover and big MTU

[Alice Mikityanska]

From: Alice Mikityanska <alice@isovalent.com>

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>
---
 tools/testing/selftests/net/Makefile         |   1 +
 tools/testing/selftests/net/cork_fragsize.py | 140 +++++++++++++++++++
 2 files changed, 141 insertions(+)
 create mode 100755 tools/testing/selftests/net/cork_fragsize.py

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index 0f5c178bc224..c6d332c90a54 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/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 \
diff --git a/tools/testing/selftests/net/cork_fragsize.py b/tools/testing/selftests/net/cork_fragsize.py
new file mode 100755
index 000000000000..c54af8f21e3b
--- /dev/null
+++ b/tools/testing/selftests/net/cork_fragsize.py
@@ -0,0 +1,140 @@
+#!/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 KsftSkipEx, ksft_eq, ksft_raises, ksft_true
+from lib.py import KsftNamedVariant, ksft_exit, ksft_pr, ksft_run, ksft_variants
+from lib.py import NetNS, NetNSEnter, ip
+
+
+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:
+    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:
+    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:
+    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:
+    try:
+        ip('link add dummy type 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
+    finally:
+        ip('link del dummy', ns=ns)
+
+
+@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:
+    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:
+    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()
-- 
2.55.0

[PATCH net v4 4/4] net: ipv6: Clamp to IP6_MAX_MTU in ip6_dst_mtu_maybe_forward

[Alice Mikityanska]

From: Alice Mikityanska <alice@isovalent.com>

Commit 427faee167bc ("net: ipv6: introduce ip6_dst_mtu_maybe_forward")
dropped the IP6_MAX_MTU clamp that used to be present in ip6_mtu(). A
similar IPv4 commit ac6627a28dbf ("net: ipv4: Consolidate ipv4_mtu and
ip_dst_mtu_maybe_forward") preserves the IP_MAX_MTU clamp.

Restore the upper bound in the IPv6 flow to avoid potential 16-bit
overflows in forwarding paths.

Fixes: 427faee167bc ("net: ipv6: introduce ip6_dst_mtu_maybe_forward")
Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
---
 include/net/ip6_route.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index c69f1c871922..b9e8d2b759e9 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -384,6 +384,8 @@ static inline unsigned int ip6_dst_mtu_maybe_forward(const struct dst_entry *dst
 	rcu_read_unlock();
 
 out:
+	mtu = min_t(unsigned int, mtu, IP6_MAX_MTU);
+
 	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
 }
 
-- 
2.55.0

Re: [PATCH net v4 3/4] selftests: net: Test UDP length overflow with PMTU discover and big MTU

[Willem de Bruijn]

Alice Mikityanska wrote:
> From: Alice Mikityanska <alice@isovalent.com>
> 
> 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>

Thanks for respinning for the code dedup between ipv4 and ipv6
> ---
>  tools/testing/selftests/net/Makefile         |   1 +
>  tools/testing/selftests/net/cork_fragsize.py | 140 +++++++++++++++++++
>  2 files changed, 141 insertions(+)
>  create mode 100755 tools/testing/selftests/net/cork_fragsize.py
> 
> diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
> index 0f5c178bc224..c6d332c90a54 100644
> --- a/tools/testing/selftests/net/Makefile
> +++ b/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 \
> diff --git a/tools/testing/selftests/net/cork_fragsize.py b/tools/testing/selftests/net/cork_fragsize.py
> new file mode 100755
> index 000000000000..c54af8f21e3b
> --- /dev/null
> +++ b/tools/testing/selftests/net/cork_fragsize.py
> @@ -0,0 +1,140 @@
> +#!/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 KsftSkipEx, ksft_eq, ksft_raises, ksft_true
> +from lib.py import KsftNamedVariant, ksft_exit, ksft_pr, ksft_run, ksft_variants
> +from lib.py import NetNS, NetNSEnter, ip
> +
> +
> +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:
> +    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:
> +    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:
> +    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

Interesting approach. In ksft the built-in defer() mechanism may be
preferred. Not sure.

> +def dummy_netdev(ns: NetNS, mtu: int, ipv6: bool) -> None:
> +    try:
> +        ip('link add dummy type 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
> +    finally:
> +        ip('link del dummy', ns=ns)
> +
> +

Re: [PATCH net v4 3/4] selftests: net: Test UDP length overflow with PMTU discover and big MTU

[Jakub Kicinski]

On Wed, 26 Aug 2026 13:18:18 -0400 Willem de Bruijn wrote:
> > +def check_dmesg_clean(func: str) -> bool:
> > +    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  
> 
> Interesting approach. In ksft the built-in defer() mechanism may be
> preferred. Not sure.

+1 small preference towards defer() if Alice is willing.
It's be more idiomatic for our selftests

More importantly ruff check and pylink --disable=R have to be clean
for new tests. The test cases need a docstring explaining what they are
trying to test. The helpers can probably get a _ prefix to mark them
as private and silence the warning.
-- 
pw-bot: cr