netfilter: nft_payload: sanitize offset and length before calling skb_checksum()

If access to offset + length is larger than the skbuff length, then
skb_checksum() triggers BUG_ON().

skb_checksum() internally subtracts the length parameter while iterating
over skbuff, BUG_ON(len) at the end of it checks that the expected
length to be included in the checksum calculation is fully consumed.

Fixes: 7ec3f7b ("netfilter: nft_payload: add packet mangling support")
Reported-by: Slavin Liu <[email protected]>
Signed-off-by: Pablo Neira Ayuso <[email protected]>

Author: ummakynes

net/netfilter/nft_payload.c

@@ -904,6 +904,9 @@ static void nft_payload_set_eval(const struct nft_expr *expr,
 	    ((priv->base != NFT_PAYLOAD_TRANSPORT_HEADER &&
 	      priv->base != NFT_PAYLOAD_INNER_HEADER) ||
 	     skb->ip_summed != CHECKSUM_PARTIAL)) {
+		if (offset + priv->len > skb->len)
+			goto err;
+
 		fsum = skb_checksum(skb, offset, priv->len, 0);
 		tsum = csum_partial(src, priv->len, 0);