ipv6: sr: fix missing sk_buff release in seg6_input_core

The seg6_input() function is responsible for adding the SRH into a
packet, delegating the operation to the seg6_input_core(). This function
uses the skb_cow_head() to ensure that there is sufficient headroom in
the sk_buff for accommodating the link-layer header.
In the event that the skb_cow_header() function fails, the
seg6_input_core() catches the error but it does not release the sk_buff,
which will result in a memory leak.

This issue was introduced in commit af3b5158 ("ipv6: sr: fix BUG due
to headroom too small after SRH push") and persists even after commit
7a3f5b0d ("netfilter: add netfilter hooks to SRv6 data plane"),
where the entire seg6_input() code was refactored to deal with netfilter
hooks.

The proposed patch addresses the identified memory leak by requiring the
seg6_input_core() function to release the sk_buff in the event that
skb_cow_head() fails.

Fixes: af3b5158 ("ipv6: sr: fix BUG due to headroom too small after SRH push")
Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/seg6_iptunnel.c

@@ -459,10 +459,8 @@ static int seg6_input_core(struct net *net, struct sock *sk,
 	int err;
 
 	err = seg6_do_srh(skb);
-	if (unlikely(err)) {
-		kfree_skb(skb);
-		return err;
-	}
+	if (unlikely(err))
+		goto drop;
 
 	slwt = seg6_lwt_lwtunnel(orig_dst->lwtstate);
 
@@ -486,7 +484,7 @@ static int seg6_input_core(struct net *net, struct sock *sk,
 
 	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
 	if (unlikely(err))
-		return err;
+		goto drop;
 
 	if (static_branch_unlikely(&nf_hooks_lwtunnel_enabled))
 		return NF_HOOK(NFPROTO_IPV6, NF_INET_LOCAL_OUT,
@@ -494,6 +492,9 @@ static int seg6_input_core(struct net *net, struct sock *sk,
 			       skb_dst(skb)->dev, seg6_input_finish);
 
 	return seg6_input_finish(dev_net(skb->dev), NULL, skb);
+drop:
+	kfree_skb(skb);
+	return err;
 }
 
 static int seg6_input_nf(struct sk_buff *skb)