Commit message: netfilter: conntrack: Make global sysctls readonly in non-init netns

These sysctls point to global variables:
- NF_SYSCTL_CT_MAX (&nf_conntrack_max)
- NF_SYSCTL_CT_EXPECT_MAX (&nf_ct_expect_max)
- NF_SYSCTL_CT_BUCKETS (&nf_conntrack_htable_size_user)

Because their data pointers are not updated to point to per-netns
structures, they must be marked read-only in a non-init_net ns.
Otherwise, changes in any net namespace are reflected in (leaked into)
all other net namespaces. This problem has existed since the
introduction of net namespaces.

The current logic marks them read-only only if the net namespace is
owned by an unprivileged user (other than init_user_ns).

Commit d0febd81ae77 ("netfilter: conntrack: re-visit sysctls in
unprivileged namespaces") "exposes all sysctls even if the namespace is
unpriviliged." Since we need to mark them readonly in any case, we can
forego the unprivileged user check altogether.

Fixes: d0febd81ae77 ("netfilter: conntrack: re-visit sysctls in unprivileged namespaces")
Signed-off-by: Jonathon Reinhart <Jonathon.Reinhart@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
CVE: CVE-2021-38209
CWE: CWE-203
Vulnerability description: net/netfilter/nf_conntrack_standalone.c in the Linux kernel before 5.12.2 allows observation of changes in any net namespace because these changes are leaked into all other net namespaces. This is related to the NF_SYSCTL_CT_MAX, NF_SYSCTL_CT_EXPECT_MAX, and NF_SYSCTL_CT_BUCKETS sysctls.

Project: linux
File: nf_conntrack_standalone.c

static int nf_conntrack_standalone_init_sysctl(struct net *net)
{
	struct nf_conntrack_net *cnet = net_generic(net, nf_conntrack_net_id);
	struct nf_udp_net *un = nf_udp_pernet(net);
	struct ctl_table *table;

	BUILD_BUG_ON(ARRAY_SIZE(nf_ct_sysctl_table) != NF_SYSCTL_CT_LAST_SYSCTL);

	table = kmemdup(nf_ct_sysctl_table, sizeof(nf_ct_sysctl_table),
			GFP_KERNEL);
	if (!table)
		return -ENOMEM;

	table[NF_SYSCTL_CT_COUNT].data = &net->ct.count;
	table[NF_SYSCTL_CT_CHECKSUM].data = &net->ct.sysctl_checksum;
	table[NF_SYSCTL_CT_LOG_INVALID].data = &net->ct.sysctl_log_invalid;
	table[NF_SYSCTL_CT_ACCT].data = &net->ct.sysctl_acct;
	table[NF_SYSCTL_CT_HELPER].data = &net->ct.sysctl_auto_assign_helper;
#ifdef CONFIG_NF_CONNTRACK_EVENTS
	table[NF_SYSCTL_CT_EVENTS].data = &net->ct.sysctl_events;
#endif
#ifdef CONFIG_NF_CONNTRACK_TIMESTAMP
	table[NF_SYSCTL_CT_TIMESTAMP].data = &net->ct.sysctl_tstamp;
#endif
	table[NF_SYSCTL_CT_PROTO_TIMEOUT_GENERIC].data = &nf_generic_pernet(net)->timeout;
	table[NF_SYSCTL_CT_PROTO_TIMEOUT_ICMP].data = &nf_icmp_pernet(net)->timeout;
	table[NF_SYSCTL_CT_PROTO_TIMEOUT_ICMPV6].data = &nf_icmpv6_pernet(net)->timeout;
	table[NF_SYSCTL_CT_PROTO_TIMEOUT_UDP].data = &un->timeouts[UDP_CT_UNREPLIED];
	table[NF_SYSCTL_CT_PROTO_TIMEOUT_UDP_STREAM].data = &un->timeouts[UDP_CT_REPLIED];

	nf_conntrack_standalone_init_tcp_sysctl(net, table);
	nf_conntrack_standalone_init_sctp_sysctl(net, table);
	nf_conntrack_standalone_init_dccp_sysctl(net, table);
	nf_conntrack_standalone_init_gre_sysctl(net, table);

	/* Don't allow unprivileged users to alter certain sysctls */
	if (net->user_ns != &init_user_ns) {
		table[NF_SYSCTL_CT_MAX].mode = 0444;
		table[NF_SYSCTL_CT_EXPECT_MAX].mode = 0444;
		table[NF_SYSCTL_CT_HELPER].mode = 0444;
#ifdef CONFIG_NF_CONNTRACK_EVENTS
		table[NF_SYSCTL_CT_EVENTS].mode = 0444;
#endif
		table[NF_SYSCTL_CT_BUCKETS].mode = 0444;
	} else if (!net_eq(&init_net, net)) {
		table[NF_SYSCTL_CT_BUCKETS].mode = 0444;
	}

	cnet->sysctl_header = register_net_sysctl(net, "net/netfilter", table);
	if (!cnet->sysctl_header)
		goto out_unregister_netfilter;

	return 0;

out_unregister_netfilter:
	kfree(table);
	return -ENOMEM;
}