net: nsh: Use correct mac_offset to unwind gso skb in nsh_gso_segment()

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

net: nsh: Use correct mac_offset to unwind gso skb in nsh_gso_segment()

As the call trace shows, skb_panic was caused by wrong skb->mac_header

in nsh_gso_segment():

invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI

CPU: 3 PID: 2737 Comm: syz Not tainted 6.3.0-next-20230505 #1

RIP: 0010:skb_panic+0xda/0xe0

call Trace:

skb_push+0x91/0xa0

nsh_gso_segment+0x4f3/0x570

skb_mac_gso_segment+0x19e/0x270

__skb_gso_segment+0x1e8/0x3c0

validate_xmit_skb+0x452/0x890

validate_xmit_skb_list+0x99/0xd0

sch_direct_xmit+0x294/0x7c0

__dev_queue_xmit+0x16f0/0x1d70

packet_xmit+0x185/0x210

packet_snd+0xc15/0x1170

packet_sendmsg+0x7b/0xa0

sock_sendmsg+0x14f/0x160

The root cause is:

nsh_gso_segment() use skb->network_header - nhoff to reset mac_header

in skb_gso_error_unwind() if inner-layer protocol gso fails.

However, skb->network_header may be reset by inner-layer protocol

gso function e.g. mpls_gso_segment. skb->mac_header reset by the

inaccurate network_header will be larger than skb headroom.

nsh_gso_segment

nhoff = skb->network_header - skb->mac_header;

__skb_pull(skb,nsh_len)

skb_mac_gso_segment

mpls_gso_segment

skb_reset_network_header(skb);//skb->network_header+=nsh_len

return -EINVAL;

skb_gso_error_unwind

skb_push(skb, nsh_len);

skb->mac_header = skb->network_header - nhoff;

// skb->mac_header > skb->headroom, cause skb_push panic

Use correct mac_offset to restore mac_header and get rid of nhoff.