Re: [PATCH xfrm] xfrm: iptfs: fix use-after-free on first_skb in __input_process_payload

[Simon Horman <horms@kernel.org>]

On Tue, May 26, 2026 at 06:53:28PM +0800, Zhenghang Xiao wrote:
> __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).
> 
> Fixes: 3f3339885fb3 ("xfrm: iptfs: add reusing received skb for the tunnel egress packet")
> Signed-off-by: Zhenghang Xiao <kipreyyy@gmail.com>

FTR: An AI generated review of this patch is available on sashiko.dev.
I believe that review can be treated in the context of possible follow-up
and should not effect the progress of this patch.