[PATCH net v4 2/4] net: ipv6: 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 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