netfilter: nf_ct_irc: cap packet search space to 4k

This uses a pseudo-linearization scheme with a 64k global buffer,
but BIG TCP arrival means IPv6 TCP stack can generate skbs
that exceed this size.

In practice, IRC commands are not expected to exceed 512 bytes, plus
this is interactive protocol, so we should not see large packets
in practice.

Given most IRC connections nowadays use TLS so this helper could also be
removed in the near future.

Fixes: 7c4e983c4f3c ("net: allow gso_max_size to exceed 65536")
Fixes: 0fe79f28bfaf ("net: allow gro_max_size to exceed 65536")
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_conntrack_irc.c

···
 EXPORT_SYMBOL_GPL(nf_nat_irc_hook);
 
 #define HELPER_NAME "irc"
+#define MAX_SEARCH_SIZE	4095
 
 MODULE_AUTHOR("Harald Welte <[e-mail]>");
 MODULE_DESCRIPTION("IRC (DCC) connection tracking helper");
···
 	int i, ret = NF_ACCEPT;
 	char *addr_beg_p, *addr_end_p;
 	typeof(nf_nat_irc_hook) nf_nat_irc;
+	unsigned int datalen;
 
 	/* If packet is coming from IRC server */
 	if (dir == IP_CT_DIR_REPLY)
···
 	if (dataoff >= skb->len)
 		return NF_ACCEPT;
 
+	datalen = skb->len - dataoff;
+	if (datalen > MAX_SEARCH_SIZE)
+		datalen = MAX_SEARCH_SIZE;
+
 	spin_lock_bh(&irc_buffer_lock);
-	ib_ptr = skb_header_pointer(skb, dataoff, skb->len - dataoff,
+	ib_ptr = skb_header_pointer(skb, dataoff, datalen,
 				    irc_buffer);
 	if (!ib_ptr) {
 		spin_unlock_bh(&irc_buffer_lock);
···
 	}
 
 	data = ib_ptr;
-	data_limit = ib_ptr + skb->len - dataoff;
+	data_limit = ib_ptr + datalen;
 
 	/* strlen("\1DCC SENT t AAAAAAAA P\1\n")=24
 	 * 5+MINMATCHLEN+strlen("t AAAAAAAA P\1\n")=14 */
···
 	irc_exp_policy.max_expected = max_dcc_channels;
 	irc_exp_policy.timeout = dcc_timeout;
 
-	irc_buffer = kmalloc(65536, GFP_KERNEL);
+	irc_buffer = kmalloc(MAX_SEARCH_SIZE + 1, GFP_KERNEL);
 	if (!irc_buffer)
 		return -ENOMEM;