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

[Alice Mikityanska <alice.kernel@fastmail.im>]

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