netfilter: nf_ct_ftp: prefer skb_linearize

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.

Use skb_linearize. It should be possible to rewrite this to properly
deal with segmented skbs (i.e., only do small chunk-wise accesses),
but this is going to be a lot more intrusive than this because every
helper function needs to get the sk_buff instead of a pointer to a raw
data buffer.

In practice, provided we're really looking at FTP control channel packets,
there should never be a case where we deal with huge packets.

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_ftp.c

···
 MODULE_ALIAS("ip_conntrack_ftp");
 MODULE_ALIAS_NFCT_HELPER(HELPER_NAME);
 
-/* This is slow, but it's simple. --RR */
-static char *ftp_buffer;
-
-static DEFINE_SPINLOCK(nf_ftp_lock);
-
 #define MAX_PORTS 8
 static u_int16_t ports[MAX_PORTS];
 static unsigned int ports_c;
···
 		return NF_ACCEPT;
 	}
 
+	if (unlikely(skb_linearize(skb)))
+		return NF_DROP;
+
 	th = skb_header_pointer(skb, protoff, sizeof(_tcph), &_tcph);
 	if (th == NULL)
 		return NF_ACCEPT;
···
 	}
 	datalen = skb->len - dataoff;
 
-	spin_lock_bh(&nf_ftp_lock);
-	fb_ptr = skb_header_pointer(skb, dataoff, datalen, ftp_buffer);
-	if (!fb_ptr) {
-		spin_unlock_bh(&nf_ftp_lock);
-		return NF_ACCEPT;
-	}
+	spin_lock_bh(&ct->lock);
+	fb_ptr = skb->data + dataoff;
 
 	ends_in_nl = (fb_ptr[datalen - 1] == '\n');
 	seq = ntohl(th->seq) + datalen;
···
 	if (ends_in_nl)
 		update_nl_seq(ct, seq, ct_ftp_info, dir, skb);
  out:
-	spin_unlock_bh(&nf_ftp_lock);
+	spin_unlock_bh(&ct->lock);
 	return ret;
 }
 
···
 static void __exit nf_conntrack_ftp_fini(void)
 {
 	nf_conntrack_helpers_unregister(ftp, ports_c * 2);
-	kfree(ftp_buffer);
 }
 
 static int __init nf_conntrack_ftp_init(void)
···
 	int i, ret = 0;
 
 	NF_CT_HELPER_BUILD_BUG_ON(sizeof(struct nf_ct_ftp_master));
-
-	ftp_buffer = kmalloc(65536, GFP_KERNEL);
-	if (!ftp_buffer)
-		return -ENOMEM;
 
 	if (ports_c == 0)
 		ports[ports_c++] = FTP_PORT;
···
 	ret = nf_conntrack_helpers_register(ftp, ports_c * 2);
 	if (ret < 0) {
 		pr_err("failed to register helpers\n");
-		kfree(ftp_buffer);
 		return ret;
 	}