[PATCH net 1/1] ipv6: fix use-after-free in ip6_finish_output2()

From: Luxiao Xu <[email protected]>

ip6_finish_output2() caches a pointer to the IPv6 destination
address (daddr) before invoking lwtunnel_xmit().  The LWT-BPF
transmit path or other encapsulation operations within
lwtunnel_xmit() can reallocate the skb head, freeing the memory
that daddr points to.  When lwtunnel_xmit() returns
LWTUNNEL_XMIT_CONTINUE, the function continues to use the stale
daddr pointer to compute the nexthop and to look up or create the
neighbour entry.  This results in a use-after-free read, which can
leak sensitive kernel data, pollute the neighbour table with
arbitrary values, misdirect traffic, or crash the system.

Fix this by re-fetching the IPv6 header and the destination
address pointer after lwtunnel_xmit() returns
LWTUNNEL_XMIT_CONTINUE, ensuring that the subsequent nexthop
computation and neighbour lookup operate on valid memory.

Fixes: e415ed3a4b8b ("ipv6: use skb_expand_head in ip6_finish_output2")
Cc: [email protected]
Reported-by: Vega <[email protected]>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Luxiao Xu <[email protected]>
Signed-off-by: Ren Wei <[email protected]>
---
 net/ipv6/ip6_output.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 368e4fa3b43c..d893c3dacfd6 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -116,6 +116,8 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 
 		if (res != LWTUNNEL_XMIT_CONTINUE)
 			return res;
+		hdr = ipv6_hdr(skb);
+		daddr = &hdr->daddr;
 	}
 
 	IP6_UPD_PO_STATS(net, idev, IPSTATS_MIB_OUT, skb->len);
-- 
2.43.0