bpf: Use nla_ok() instead of checking nla_len directly

nla_len may also be too short to be sane, in which case after
recent changes nla_len() will return a wrapped value.

Fixes: 172db56 ("netlink: Return unsigned value for nla_len()")
Reported-by: [email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: kuba-moo

net/core/filter.c

@@ -203,7 +203,7 @@ BPF_CALL_3(bpf_skb_get_nlattr_nest, struct sk_buff *, skb, u32, a, u32, x)
 		return 0;
 
 	nla = (struct nlattr *) &skb->data[a];
-	if (nla->nla_len > skb->len - a)
+	if (!nla_ok(nla, skb->len - a))
 		return 0;
 
 	nla = nla_find_nested(nla, x);