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).

Fixes: 3f3339885fb3 ("xfrm: iptfs: add reusing received skb for the tunnel egress packet")
Signed-off-by: Zhenghang Xiao <kipreyyy@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_iptfs.c

diff --git a/net/xfrm/xfrm_iptfs.c b/net/xfrm/xfrm_iptfs.c
index 62ba828..aea63a0 100644
--- a/net/xfrm/xfrm_iptfs.c
+++ b/net/xfrm/xfrm_iptfs.c
@@ -954,6 +954,7 @@ static bool __input_process_payload(struct xfrm_state *x, u32 data,
 	u32 first_iplen, iphlen, iplen, remaining, tail;
 	u32 capturelen;
 	u64 seq;
+	bool first_skb_partial = false;
 
 	xtfs = x->mode_data;
 	net = xs_net(x);
@@ -1161,6 +1162,7 @@ static bool __input_process_payload(struct xfrm_state *x, u32 data,
 
 			spin_unlock(&xtfs->drop_lock);
 
+			first_skb_partial = (first_skb == skb);
 			break;
 		}
 
@@ -1172,7 +1174,7 @@ static bool __input_process_payload(struct xfrm_state *x, u32 data,
 		/* this should not happen from the above code */
 		XFRM_INC_STATS(net, LINUX_MIB_XFRMINIPTFSERROR);
 
-	if (first_skb && first_iplen && !defer && first_skb != xtfs->ra_newskb) {
+	if (first_skb && first_iplen && !defer && !first_skb_partial) {
 		/* first_skb is queued b/c !defer and not partial */
 		if (pskb_trim(first_skb, first_iplen)) {
 			/* error trimming */