bpf: Check negative offsets in __bpf_skb_min_len()

skb_network_offset() and skb_transport_offset() can be negative when
they are called after we pull the transport header, for example, when
we use eBPF sockmap at the point of ->sk_data_ready().

__bpf_skb_min_len() uses an unsigned int to get these offsets, this
leads to a very large number which then causes bpf_skb_change_tail()
failed unexpectedly.

Fix this by using a signed int to get these offsets and ensure the
minimum is at least zero.

Fixes: 5293efe ("bpf: add bpf_skb_change_tail helper")
Signed-off-by: Cong Wang <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: John Fastabend <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: Cong Wang

net/core/filter.c

@@ -3734,13 +3734,22 @@ static const struct bpf_func_proto bpf_skb_adjust_room_proto = {
 
 static u32 __bpf_skb_min_len(const struct sk_buff *skb)
 {
-	u32 min_len = skb_network_offset(skb);
+	int offset = skb_network_offset(skb);
+	u32 min_len = 0;
 
-	if (skb_transport_header_was_set(skb))
-		min_len = skb_transport_offset(skb);
-	if (skb->ip_summed == CHECKSUM_PARTIAL)
-		min_len = skb_checksum_start_offset(skb) +
-			  skb->csum_offset + sizeof(__sum16);
+	if (offset > 0)
+		min_len = offset;
+	if (skb_transport_header_was_set(skb)) {
+		offset = skb_transport_offset(skb);
+		if (offset > 0)
+			min_len = offset;
+	}
+	if (skb->ip_summed == CHECKSUM_PARTIAL) {
+		offset = skb_checksum_start_offset(skb) +
+			 skb->csum_offset + sizeof(__sum16);
+		if (offset > 0)
+			min_len = offset;
+	}
 	return min_len;
 }