reload inner IPv6 header after GSO offloads

Summary

In the Linux kernel, the following vulnerability has been resolved: ipv6: sit: reload inner IPv6 header after GSO offloads ipip6_tunnel_xmit() caches the inner IPv6 header pointer at function entry and continues using it after iptunnel_handle_offloads(). For GSO skbs, iptunnel_handle_offloads() calls skb_header_unclone(). When the skb header is cloned, skb_header_unclone() can call pskb_expand_head(), which may move the skb head. The pskb_expand_head() contract requires pointers into the skb header to be reloaded after the call. If the later skb_realloc_headroom() branch is not taken, SIT uses the stale iph6 pointer to read the inner hop limit and DS field. That can read from a freed skb head after the old head's remaining clone is released. Reload iph6 after the offload helper succeeds and before subsequent reads from the inner IPv6 header. Keep the existing reload after skb_realloc_headroom(), since that branch can also replace the skb. A flaw was found in the Linux kernel's Simple Internet Transition (SIT) tunnel driver for IPv6. When processing network traffic with Generic Segmentation Offload (GSO) enabled, the driver may use a stale pointer to the inner IPv6 header after the socket buffer (skb) head has been reallocated. This can lead to reading from freed memory, potentially allowing an attacker to cause a denial of service or gain access to sensitive information.