[PATCH 05/12] ipv6: xfrm: use full sockets in local error paths

[Steffen Klassert <steffen.klassert@secunet.com>]

From: Zhiling Zou <zhilinz@nebusec.ai>

xfrm6_local_rxpmtu() and xfrm6_local_error() dereference skb->sk as if it
always pointed at a full IPv6 socket.

That is not guaranteed. TCP SYN-ACK skbs can be owned by a
TCP_NEW_SYN_RECV request_sock while the output path itself is driven by the
full listener. If rerouting selects an IPv6 XFRM tunnel route with a lower
MTU, the local PMTU/error handling path can reach these callbacks with that
mini-socket still attached to the skb.

The callbacks then miscast the request socket as a full inet/IPv6 socket and
can read beyond the request_sock allocation when they access inet_sock or
ipv6_pinfo state.

Resolve the owner with skb_to_full_sk() in both callbacks and bail out when
no full socket is attached. This matches the surrounding XFRM IPv6 PMTU/error
logic, which already reasons about full sockets with skb_to_full_sk().

Fixes: dd767856a36e ("xfrm6: Don't call icmpv6_send on local error")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
---
 net/ipv6/xfrm6_output.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/xfrm6_output.c b/net/ipv6/xfrm6_output.c
index 512bdaf13699..44b221a09a0c 100644
--- a/net/ipv6/xfrm6_output.c
+++ b/net/ipv6/xfrm6_output.c
@@ -19,7 +19,10 @@
 void xfrm6_local_rxpmtu(struct sk_buff *skb, u32 mtu)
 {
 	struct flowi6 fl6;
-	struct sock *sk = skb->sk;
+	struct sock *sk = skb_to_full_sk(skb);
+
+	if (!sk)
+		return;
 
 	fl6.flowi6_oif = sk->sk_bound_dev_if;
 	fl6.daddr = ipv6_hdr(skb)->daddr;
@@ -31,7 +34,10 @@ void xfrm6_local_error(struct sk_buff *skb, u32 mtu)
 {
 	struct flowi6 fl6;
 	const struct ipv6hdr *hdr;
-	struct sock *sk = skb->sk;
+	struct sock *sk = skb_to_full_sk(skb);
+
+	if (!sk)
+		return;
 
 	hdr = skb->encapsulation ? inner_ipv6_hdr(skb) : ipv6_hdr(skb);
 	fl6.fl6_dport = inet_sk(sk)->inet_dport;
-- 
2.43.0