netfilter: nft_exthdr: Search chunks in SCTP packets only

Since user space does not generate a payload dependency, plain sctp
chunk matches cause searching in non-SCTP packets, too. Avoid this
potential mis-interpretation of packet data by checking pkt->tprot.

Fixes: 133dc203d7 ("netfilter: nft_exthdr: Support SCTP chunks")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_exthdr.c

@@ -312,6 +312,9 @@ static void nft_exthdr_sctp_eval(const struct nft_expr *expr,
 	const struct sctp_chunkhdr *sch;
 	struct sctp_chunkhdr _sch;
 
+	if (pkt->tprot != IPPROTO_SCTP)
+		goto err;
+
 	do {
 		sch = skb_header_pointer(pkt->skb, offset, sizeof(_sch), &_sch);
 		if (!sch || !sch->length)
@@ -334,7 +337,7 @@ static void nft_exthdr_sctp_eval(const struct nft_expr *expr,
 		}
 		offset += SCTP_PAD4(ntohs(sch->length));
 	} while (offset < pkt->skb->len);
-
+err:
 	if (priv->flags & NFT_EXTHDR_F_PRESENT)
 		nft_reg_store8(dest, false);
 	else