netfilter: nftables: allow re-computing sctp CRC-32C in 'payload' statements

nftables payload statements are used to mangle SCTP headers, but they can
only replace the Internet Checksum. As a consequence, nftables rules that
mangle sport/dport/vtag in SCTP headers potentially generate packets that
are discarded by the receiver, unless the CRC-32C is "offloaded" (e.g the
rule mangles a skb having 'ip_summed' equal to 'CHECKSUM_PARTIAL'.

Fix this extending uAPI definitions and L4 checksum update function, in a
way that userspace programs (e.g. nft) can instruct the kernel to compute
CRC-32C in SCTP headers. Also ensure that LIBCRC32C is built if NF_TABLES
is 'y' or 'm' in the kernel build configuration.

Signed-off-by: Davide Caratti <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

include/uapi/linux/netfilter/nf_tables.h

···
  *
  * @NFT_PAYLOAD_CSUM_NONE: no checksumming
  * @NFT_PAYLOAD_CSUM_INET: internet checksum (RFC 791)
+ * @NFT_PAYLOAD_CSUM_SCTP: CRC-32c, for use in SCTP header (RFC 3309)
  */
 enum nft_payload_csum_types {
 	NFT_PAYLOAD_CSUM_NONE,
 	NFT_PAYLOAD_CSUM_INET,
+	NFT_PAYLOAD_CSUM_SCTP,
 };
 
 enum nft_payload_csum_flags {

net/netfilter/Kconfig

···
 
 config NF_TABLES
 	select NETFILTER_NETLINK
+	select LIBCRC32C
 	tristate "Netfilter nf_tables support"
 	help
 	  nftables is the new packet classification framework that intends to

net/netfilter/nft_payload.c

···
 #include <linux/icmpv6.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>
+#include <net/sctp/checksum.h>
 
 static bool nft_payload_rebuild_vlan_hdr(const struct sk_buff *skb, int mac_off,
 					 struct vlan_ethhdr *veth)
···
 	return 0;
 }
 
+static int nft_payload_csum_sctp(struct sk_buff *skb, int offset)
+{
+	struct sctphdr *sh;
+
+	if (skb_ensure_writable(skb, offset + sizeof(*sh)))
+		return -1;
+
+	sh = (struct sctphdr *)(skb->data + offset);
+	sh->checksum = sctp_compute_cksum(skb, offset);
+	skb->ip_summed = CHECKSUM_UNNECESSARY;
+	return 0;
+}
+
 static int nft_payload_l4csum_update(const struct nft_pktinfo *pkt,
 				     struct sk_buff *skb,
 				     __wsum fsum, __wsum tsum)
···
 	    skb_store_bits(skb, offset, src, priv->len) < 0)
 		goto err;
 
+	if (priv->csum_type == NFT_PAYLOAD_CSUM_SCTP &&
+	    pkt->tprot == IPPROTO_SCTP &&
+	    skb->ip_summed != CHECKSUM_PARTIAL) {
+		if (nft_payload_csum_sctp(skb, pkt->xt.thoff))
+			goto err;
+	}
+
 	return;
 err:
 	regs->verdict.code = NFT_BREAK;
···
 	switch (priv->csum_type) {
 	case NFT_PAYLOAD_CSUM_NONE:
 	case NFT_PAYLOAD_CSUM_INET:
+		break;
+	case NFT_PAYLOAD_CSUM_SCTP:
+		if (priv->base != NFT_PAYLOAD_TRANSPORT_HEADER)
+			return -EINVAL;
+
+		if (priv->csum_offset != offsetof(struct sctphdr, checksum))
+			return -EINVAL;
 		break;
 	default:
 		return -EOPNOTSUPP;