xfrm: iptfs: fix use-after-free on first_skb in __input_process_payload

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

xfrm: iptfs: fix use-after-free on first_skb in __input_process_payload

__input_process_payload() stores first_skb into xtfs->ra_newskb under

drop_lock when starting partial reassembly, then unlocks and breaks out

of the processing loop. The post-loop check reads xtfs->ra_newskb

without the lock to decide whether first_skb is still owned:

if (first_skb && first_iplen && !defer && first_skb != xtfs->ra_newskb)

Between spin_unlock and this read, a concurrent CPU running

iptfs_reassem_cont() (or the drop_timer hrtimer) can complete

reassembly, NULL xtfs->ra_newskb, and free the skb. The check then

evaluates first_skb != NULL as true, and pskb_trim/ip_summed/consume_skb

operate on the freed skb — a use-after-free in skbuff_head_cache.

Replace the unlocked read with a local bool that records whether

first_skb was handed to the reassembly state in the current call. The

flag is set after the existing spin_unlock, before the break, using the

pointer equality that is stable at that point (first_skb == skb iff

first_skb was stored in ra_newskb).