Linux: net: ipv6: Fix UDP length overflow with PMTU discover and big MTU

In the Linux kernel, the following vulnerability has been resolved:

net: ipv6: Fix UDP length overflow with PMTU discover and big MTU

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.

1. 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.

1. 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).

1. 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").