bpf: Verify flags in bpf_fib_lookup

Verify flags argument contains only known flags. Allows programs to probe
for support as more are added.
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/filter.c

@@ -4270,6 +4270,9 @@ BPF_CALL_4(bpf_xdp_fib_lookup, struct xdp_buff *, ctx,
 	if (plen < sizeof(*params))
 		return -EINVAL;
 
+	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))
+		return -EINVAL;
+
 	switch (params->family) {
 #if IS_ENABLED(CONFIG_INET)
 	case AF_INET:
@@ -4304,6 +4307,9 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 	if (plen < sizeof(*params))
 		return -EINVAL;
 
+	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))
+		return -EINVAL;
+
 	switch (params->family) {
 #if IS_ENABLED(CONFIG_INET)
 	case AF_INET: