[NETFILTER]: nf_conntrack_expect: introduce nf_conntrack_expect_max sysct

As a last step of preventing DoS by creating lots of expectations, this
patch introduces a global maximum and a sysctl to control it. The default
is initialized to 4 * the expectation hash table size, which results in
1/64 of the default maxmimum of conntracks.

Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/netfilter/nf_conntrack_expect.h

···
 
 extern struct hlist_head *nf_ct_expect_hash;
 extern unsigned int nf_ct_expect_hsize;
+extern unsigned int nf_ct_expect_max;
 
 struct nf_conntrack_expect
 {

net/netfilter/nf_conntrack_expect.c

···
 
 static unsigned int nf_ct_expect_hash_rnd __read_mostly;
 static unsigned int nf_ct_expect_count;
+unsigned int nf_ct_expect_max __read_mostly;
 static int nf_ct_expect_hash_rnd_initted __read_mostly;
 static int nf_ct_expect_vmalloc;
 
···
 	    master_help->expecting >= master_help->helper->max_expected)
 		evict_oldest_expect(master);
 
+	if (nf_ct_expect_count >= nf_ct_expect_max) {
+		if (net_ratelimit())
+			printk(KERN_WARNING
+			       "nf_conntrack: expectation table full");
+		ret = -EMFILE;
+		goto out;
+	}
+
 	nf_ct_expect_insert(expect);
 	nf_ct_expect_event(IPEXP_NEW, expect);
 	ret = 0;
···
 		if (!nf_ct_expect_hsize)
 			nf_ct_expect_hsize = 1;
 	}
+	nf_ct_expect_max = nf_ct_expect_hsize * 4;
 
 	nf_ct_expect_hash = nf_ct_alloc_hashtable(&nf_ct_expect_hsize,
 						  &nf_ct_expect_vmalloc);

net/netfilter/nf_conntrack_standalone.c

···
 		.extra1		= &log_invalid_proto_min,
 		.extra2		= &log_invalid_proto_max,
 	},
-
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "nf_conntrack_expect_max",
+		.data		= &nf_ct_expect_max,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 	{ .ctl_name = 0 }
 };