[PATCH net] net: fou: do not use guehdr after iptunnel_pull_offloads in gue_udp_recv

[Lorenzo Bianconi]

gue tunnels run iptunnel_pull_offloads on received skbs. This can
determine a possible use-after-free accessing guehdr pointer since
the packet will be 'uncloned' running pskb_expand_head if it is a
cloned gso skb (e.g if the packet has been sent though a veth device)

Fixes: a09a4c8dd1ec ("tunnels: Remove encapsulation offloads on decap")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianc...@redhat.com>
---
 net/ipv4/fou.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/fou.c b/net/ipv4/fou.c
index 79e98e21cdd7..12ce6c526d72 100644
--- a/net/ipv4/fou.c
+++ b/net/ipv4/fou.c
@@ -121,6 +121,7 @@ static int gue_udp_recv(struct sock *sk, struct sk_buff 
*skb)
        struct guehdr *guehdr;
        void *data;
        u16 doffset = 0;
+       u8 proto_ctype;
 
        if (!fou)
                return 1;
@@ -212,13 +213,14 @@ static int gue_udp_recv(struct sock *sk, struct sk_buff 
*skb)
        if (unlikely(guehdr->control))
                return gue_control_message(skb, guehdr);
 
+       proto_ctype = guehdr->proto_ctype;
        __skb_pull(skb, sizeof(struct udphdr) + hdrlen);
        skb_reset_transport_header(skb);
 
        if (iptunnel_pull_offloads(skb))
                goto drop;
 
-       return -guehdr->proto_ctype;
+       return -proto_ctype;
 
 drop:
        kfree_skb(skb);
-- 
2.20.1