Linux: net: ipv4: Fix UDP length overflow with PMTU discover and big MTU

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

net: ipv4: Fix UDP length overflow with PMTU discover and big MTU

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.

1. 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.

1. 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).

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.

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.