netfilter: nf_tables: allow NFPROTO_INET in nft_(match/target)_validate()

Commit d0009effa886 ("netfilter: nf_tables: validate NFPROTO_* family") added some validation of NFPROTO_* families in the nft_compat module, but it broke the ability to use legacy iptables modules in dual-stack nftables.

While with legacy iptables one had to independently manage IPv4 and IPv6 tables, with nftables it is possible to have dual-stack tables sharing the rules. Moreover, it was possible to use rules based on legacy iptables match/target modules in dual-stack nftables. Consider the following program:

```

/* #define TBL_FAMILY NFPROTO_IPV4 */

/*
 * creates something like below
 * table inet testfw {
 *   chain input {
 *     type filter hook prerouting priority filter; policy accept;
 *     bytecode counter packets 0 bytes 0 accept
 *   }
 * }
 *
 * compile:
 * cc -o nftbpf nftbpf.c -lnftnl -lmnl
 */
int main(void)
{
    uint8_t buf[MNL_SOCKET_BUFFER_SIZE];
    uint32_t seq, rule_seq, portid;
    struct mnl_nlmsg_batch *batch;
	struct nlmsghdr *nlh;
	struct mnl_socket *nl;
	int ret;
	struct xt_bpf_info_v1 *xt_bpf_info = malloc(sizeof(*xt_bpf_info));
	struct nftnl_expr *m, *cnt, *im;
	struct nftnl_rule *r;
	struct nftnl_chain *c;
    struct nftnl_table *t = nftnl_table_alloc();
    if (t == NULL) {
        perror("TABLE OOM");
        exit(EXIT_FAILURE);
    }
    nftnl_table_set_u32(t, NFTNL_TABLE_FAMILY, TBL_FAMILY);
	nftnl_table_set_str(t, NFTNL_TABLE_NAME, TBL_NAME);

	c = nftnl_chain_alloc();
	if (c == NULL) {
		perror("CHAIN OOM");
		exit(EXIT_FAILURE);
	}
	nftnl_chain_set_str(c, NFTNL_CHAIN_TABLE, TBL_NAME);
	nftnl_chain_set_str(c, NFTNL_CHAIN_NAME, CHAIN_NAME);
	nftnl_chain_set_u32(c, NFTNL_CHAIN_HOOKNUM, NF_INET_PRE_ROUTING);
	nftnl_chain_set_u32(c, NFTNL_CHAIN_PRIO, 0);

	r = nftnl_rule_alloc();
	if (r == NULL) {
		perror("RULE OOM");
		exit(EXIT_FAILURE);
	}
	nftnl_rule_set_str(r, NFTNL_RULE_TABLE, TBL_NAME);
	nftnl_rule_set_str(r, NFTNL_RULE_CHAIN, CHAIN_NAME);
	nftnl_rule_set_u32(r, NFTNL_RULE_FAMILY, TBL_FAMILY);

	m = nftnl_expr_alloc("match");
	if (m == NULL) {
		perror("MATCH OOM");
		exit(EXIT_FAILURE);
	}
	nftnl_expr_set_str(m, NFTNL_EXPR_MT_NAME, "bpf");
	nftnl_expr_set_u32(m, NFTNL_EXPR_MT_REV, 1);

	if (xt_bpf_info == NULL) {
		perror("XT_BPF OOM");
		exit(EXIT_FAILURE);
	}

	/*
	 * example from https://ipset.netfilter.org/iptables-extensions.man.html
	 * should match TCP packets
	 */
	xt_bpf_info->mode = XT_BPF_MODE_BYTECODE;
	xt_bpf_info->bpf_program_num_elem = 4;

	xt_bpf_info->bpf_program[0].code = 48;
	xt_bpf_info->bpf_program[0].jt = 0;
	xt_bpf_info->bpf_program[0].jf = 0;
	xt_bpf_info->bpf_program[0].k = 9;

	xt_bpf_info->bpf_program[1].code = 21;
	xt_bpf_info->bpf_program[1].jt = 0;
	xt_bpf_info->bpf_program[1].jf = 1;
	xt_bpf_info->bpf_program[1].k = 6;

	xt_bpf_info->bpf_program[2].code = 6;
	xt_bpf_info->bpf_program[2].jt = 0;
	xt_bpf_info->bpf_program[2].jf = 0;
	xt_bpf_info->bpf_program[2].k = 1;

	xt_bpf_info->bpf_program[3].code = 6;
	xt_bpf_info->bpf_program[3].jt = 0;
	xt_bpf_info->bpf_program[3].jf = 0;
	xt_bpf_info->bpf_program[3].k = 0;

	nftnl_expr_set(m, NFTNL_EXPR_MT_INFO, xt_bpf_info, sizeof(*xt_bpf_info));
	nftnl_rule_add_expr(r, m);

	cnt = nftnl_expr_alloc("counter");
	if (cnt == NULL) {
		perror("COUNTER OOM");
		exit(EXIT_FAILURE);
	}
	nftnl_expr_set_u64(cnt, NFTNL_EXPR_CTR_PACKETS, 0);
	nftnl_expr_set_u64(cnt, NFTNL_EXPR_CTR_BYTES, 0);
	nftnl_rule_add_expr(r, cnt);

	im = nftnl_expr_alloc("immediate");
	if (im == NULL) {
		perror("IMMEDIATE OOM");
		exit(EXIT_FAILURE);
	}
	nftnl_expr_set_u32(im, NFTNL_EXPR_IMM_DREG, 0);
	nftnl_expr_set_u32(im, NFTNL_EXPR_IMM_VERDICT, 1);
	nftnl_rule_add_expr(r, im);

	seq = time(NULL);
	batch = mnl_nlmsg_batch_start(buf, sizeof(buf));

	nftnl_batch_begin(mnl_nlmsg_batch_current(batch), seq++);
	mnl_nlmsg_batch_next(batch);

	nlh = nftnl_nlmsg_build_hdr(mnl_nlmsg_batch_current(batch),
				    NFT_MSG_NEWTABLE, TBL_FAMILY,
				    NLM_F_CREATE, seq++);
	nftnl_table_nlmsg_build_payload(nlh, t);
	nftnl_table_free(t);
	mnl_nlmsg_batch_next(batch);

	nlh = nftnl_nlmsg_build_hdr(mnl_nlmsg_batch_current(batch),
				    NFT_MSG_NEWCHAIN, TBL_FAMILY,
				    NLM_F_CREATE, seq++);
	nftnl_chain_nlmsg_build_payload(nlh, c);
	nftnl_chain_free(c);
	mnl_nlmsg_batch_next(batch);

	rule_seq = seq;
	nlh = nftnl_nlmsg_build_hdr(mnl_nlmsg_batch_current(batch),
				    NFT_MSG_NEWRULE, TBL_FAMILY,
				    NLM_F_APPEND | NLM_F_CREATE | NLM_F_ACK,
				    seq++);
	nftnl_rule_nlmsg_build_payload(nlh, r);
	nftnl_rule_free(r);
	mnl_nlmsg_batch_next(batch);

	nftnl_batch_end(mnl_nlmsg_batch_current(batch), seq++);
	mnl_nlmsg_batch_next(batch);

	nftnl_batch_end(mnl_nlmsg_batch_current(batch), seq++);
	mnl_nlmsg_batch_next(batch);

	nl = mnl_socket_open(NETLINK_NETFILTER);
	if (nl == NULL) {
		perror("mnl_socket_open");
		exit(EXIT_FAILURE);
	}

	if (mnl_socket_bind(nl, 0, MNL_SOCKET_AUTOPID) < 0) {
		perror("mnl_socket_bind");
		exit(EXIT_FAILURE);
	}
	portid = mnl_socket_get_portid(nl);

	if (mnl_socket_sendto(nl, mnl_nlmsg_batch_head(batch),
			      mnl_nlmsg_batch_size(batch)) < 0) {
		perror("mnl_socket_send");
		exit(EXIT_FAILURE);
	}

	mnl_nlmsg_batch_stop(batch);

	ret = mnl_socket_recvfrom(nl, buf, sizeof(buf));
	while (ret > 0) {
		ret = mnl_cb_run(buf, ret, rule_seq, portid, NULL, NULL);
		if (ret <= 0)
			break;
		ret = mnl_socket_recvfrom(nl, buf, sizeof(buf));
	}
	if (ret == -1) {
		perror("error");
		exit(EXIT_FAILURE);
	}
	mnl_socket_close(nl);

	return EXIT_SUCCESS;
}
```

Above creates an INET dual-stack family table using xt_bpf based rule. After d0009effa886 ("netfilter: nf_tables: validate NFPROTO_* family") we get EOPNOTSUPP for the above configuration.

Fix this by allowing NFPROTO_INET for nft_(match/target)_validate(), but also restrict the functions to classic iptables hooks.

Changes in v2:
* restrict nft_(match/target)_validate() to classic iptables hooks
* rewrite example program to use unmodified libnftnl

---
 net/netfilter/nft_compat.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

index 1f9474fefe84..d3d11dede545 100644
@@ -359,10 +359,20 @@ static int nft_target_validate(const struct nft_ctx *ctx,
 
 	if (ctx->family != NFPROTO_IPV4 &&
 	    ctx->family != NFPROTO_IPV6 &&
+	    ctx->family != NFPROTO_INET &&
 	    ctx->family != NFPROTO_BRIDGE &&
 	    ctx->family != NFPROTO_ARP)
 		return -EOPNOTSUPP;
 
+	ret = nft_chain_validate_hooks(ctx->chain,
+				       (1 << NF_INET_PRE_ROUTING) |
+				       (1 << NF_INET_LOCAL_IN) |
+				       (1 << NF_INET_FORWARD) |
+				       (1 << NF_INET_LOCAL_OUT) |
+				       (1 << NF_INET_POST_ROUTING));
+	if (ret)
+		return ret;
+
 	if (nft_is_base_chain(ctx->chain)) {
 		const struct nft_base_chain *basechain =
 						nft_base_chain(ctx->chain);

@@ -610,10 +620,20 @@ static int nft_match_validate(const struct nft_ctx *ctx,
 
 	if (ctx->family != NFPROTO_IPV4 &&
 	    ctx->family != NFPROTO_IPV6 &&
+	    ctx->family != NFPROTO_INET &&
 	    ctx->family != NFPROTO_BRIDGE &&
 	    ctx->family != NFPROTO_ARP)
 		return -EOPNOTSUPP;
 
+	ret = nft_chain_validate_hooks(ctx->chain,
+				       (1 << NF_INET_PRE_ROUTING) |
+				       (1 << NF_INET_LOCAL_IN) |
+				       (1 << NF_INET_FORWARD) |
+				       (1 << NF_INET_LOCAL_OUT) |
+				       (1 << NF_INET_POST_ROUTING));
+	if (ret)
+		return ret;
+
 	if (nft_is_base_chain(ctx->chain)) {
 		const struct nft_base_chain *basechain =
 						nft_base_chain(ctx->chain);

2.39.2

Upstream nft does not provides this. Please, clarify that this the output with the out-of-tree patch,

Please, no program in the commit description, it makes it too long, I am not sure this is the good place to store this.

Thanks! Please send a v3 with updates.

What if I replace the table and chain creation with nft commands and just leave the code creating the rule? This would make the overall program shorter, but would still illustrate the example