ipv6: reject malicious packets in ipv6_gso_segment()

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

ipv6: reject malicious packets in ipv6_gso_segment()

syzbot was able to craft a packet with very long IPv6 extension headers

leading to an overflow of skb->transport_header.

This 16bit field has a limited range.

Add skb_reset_transport_header_careful() helper and use it

from ipv6_gso_segment()

WARNING: CPU: 0 PID: 5871 at ./include/linux/skbuff.h:3032 skb_reset_transport_header include/linux/skbuff.h:3032 [inline]

WARNING: CPU: 0 PID: 5871 at ./include/linux/skbuff.h:3032 ipv6_gso_segment+0x15e2/0x21e0 net/ipv6/ip6_offload.c:151

Modules linked in:

CPU: 0 UID: 0 PID: 5871 Comm: syz-executor211 Not tainted 6.16.0-rc6-syzkaller-g7abc678e3084 #0 PREEMPT(full)

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025

RIP: 0010:skb_reset_transport_header include/linux/skbuff.h:3032 [inline]

RIP: 0010:ipv6_gso_segment+0x15e2/0x21e0 net/ipv6/ip6_offload.c:151

Call Trace:

<TASK>

skb_mac_gso_segment+0x31c/0x640 net/core/gso.c:53

nsh_gso_segment+0x54a/0xe10 net/nsh/nsh.c:110

skb_mac_gso_segment+0x31c/0x640 net/core/gso.c:53

__skb_gso_segment+0x342/0x510 net/core/gso.c:124

skb_gso_segment include/net/gso.h:83 [inline]

validate_xmit_skb+0x857/0x11b0 net/core/dev.c:3950

validate_xmit_skb_list+0x84/0x120 net/core/dev.c:4000

sch_direct_xmit+0xd3/0x4b0 net/sched/sch_generic.c:329

__dev_xmit_skb net/core/dev.c:4102 [inline]

__dev_queue_xmit+0x17b6/0x3a70 net/core/dev.c:4679