bpf: fix bpf_skb_load_bytes_relative pkt length check

The len > skb_headlen(skb) cannot be used as a maximum upper bound
for the packet length since it does not have any relation to the full
linear packet length when filtering is used from upper layers (e.g.
in case of reuseport BPF programs) as by then skb->data, skb->len
already got mangled through __skb_pull() and others.

Fixes: 4e1ec56c ("bpf: add skb_load_bytes_relative helper")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>

net/core/filter.c

@@ -1712,24 +1712,26 @@ static const struct bpf_func_proto bpf_skb_load_bytes_proto = {
 BPF_CALL_5(bpf_skb_load_bytes_relative, const struct sk_buff *, skb,
 	   u32, offset, void *, to, u32, len, u32, start_header)
 {
+	u8 *end = skb_tail_pointer(skb);
+	u8 *net = skb_network_header(skb);
+	u8 *mac = skb_mac_header(skb);
 	u8 *ptr;
 
-	if (unlikely(offset > 0xffff || len > skb_headlen(skb)))
+	if (unlikely(offset > 0xffff || len > (end - mac)))
 		goto err_clear;
 
 	switch (start_header) {
 	case BPF_HDR_START_MAC:
-		ptr = skb_mac_header(skb) + offset;
+		ptr = mac + offset;
 		break;
 	case BPF_HDR_START_NET:
-		ptr = skb_network_header(skb) + offset;
+		ptr = net + offset;
 		break;
 	default:
 		goto err_clear;
 	}
 
-	if (likely(ptr >= skb_mac_header(skb) &&
-		   ptr + len <= skb_tail_pointer(skb))) {
+	if (likely(ptr >= mac && ptr + len <= end)) {
 		memcpy(to, ptr, len);
 		return 0;
 	}