netfilter: conntrack: fix false CRC32c mismatch using paged skb

sctp_compute_cksum() implementation assumes that at least the SCTP header
is in the linear part of skb: modify conntrack error callback to avoid
false CRC32c mismatch, if the transport header is partially/entirely paged.

Fixes: cf6e007eef83 ("netfilter: conntrack: validate SCTP crc32c in PREROUTING")
Signed-off-by: Davide Caratti <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_conntrack_proto_sctp.c

···
 		      u8 pf, unsigned int hooknum)
 {
 	const struct sctphdr *sh;
-	struct sctphdr _sctph;
 	const char *logmsg;
 
-	sh = skb_header_pointer(skb, dataoff, sizeof(_sctph), &_sctph);
-	if (!sh) {
+	if (skb->len < dataoff + sizeof(struct sctphdr)) {
 		logmsg = "nf_ct_sctp: short packet ";
 		goto out_invalid;
 	}
 	if (net->ct.sysctl_checksum && hooknum == NF_INET_PRE_ROUTING &&
 	    skb->ip_summed == CHECKSUM_NONE) {
+		if (!skb_make_writable(skb, dataoff + sizeof(struct sctphdr))) {
+			logmsg = "nf_ct_sctp: failed to read header ";
+			goto out_invalid;
+		}
+		sh = (const struct sctphdr *)(skb->data + dataoff);
 		if (sh->checksum != sctp_compute_cksum(skb, dataoff)) {
 			logmsg = "nf_ct_sctp: bad CRC ";
 			goto out_invalid;