nf_conntrack: avoid kernel pointer value leak in slab name

commit 31b0b385 upstream.

The slab name ends up being visible in the directory structure under
/sys, and even if you don't have access rights to the file you can see
the filenames.

Just use a 64-bit counter instead of the pointer to the 'net' structure
to generate a unique name.

This code will go away in 4.7 when the conntrack code moves to a single
kmemcache, but this is the backportable simple solution to avoiding
leaking kernel pointers to user space.

Fixes: 5b3501fa ("netfilter: nf_conntrack: per netns nf_conntrack_cachep")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/netfilter/nf_conntrack_core.c

@@ -1493,6 +1493,7 @@ static int nf_conntrack_init_init_net(void)
 
 static int nf_conntrack_init_net(struct net *net)
 {
+	static atomic64_t unique_id;
 	int ret;
 
 	atomic_set(&net->ct.count, 0);
@@ -1504,7 +1505,8 @@ static int nf_conntrack_init_net(struct net *net)
 		goto err_stat;
 	}
 
-	net->ct.slabname = kasprintf(GFP_KERNEL, "nf_conntrack_%p", net);
+	net->ct.slabname = kasprintf(GFP_KERNEL, "nf_conntrack_%llu",
+				(u64)atomic64_inc_return(&unique_id));
 	if (!net->ct.slabname) {
 		ret = -ENOMEM;
 		goto err_slabname;