netns: provide pure entropy for net_hash_mix()

commit 355b98553789b646ed97ad801a619ff898471b92 upstream.

net_hash_mix() currently uses kernel address of a struct net,
and is used in many places that could be used to reveal this
address to a patient attacker, thus defeating KASLR, for
the typical case (initial net namespace, &init_net is
not dynamically allocated)

I believe the original implementation tried to avoid spending
too many cycles in this function, but security comes first.

Also provide entropy regardless of CONFIG_NET_NS.

Fixes: 0b4419162a ("netns: introduce the net_hash_mix "salt" for hashes")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Amit Klein <aksecurity@gmail.com>
Reported-by: Benny Pinkas <benny@pinkas.net>
Cc: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
CVE-2019-10638
Signed-off-by: Kevin F. Haggerty <haggertk@lineageos.org>
Change-Id: Iefa96bd40b5918e3e878182f9de7e978ea975fb0

include/net/net_namespace.h

@@ -50,6 +50,8 @@ struct net {
 #endif
 	spinlock_t		rules_mod_lock;
 
+	u32			hash_mix;
+
 	struct list_head	list;		/* list of network namespaces */
 	struct list_head	cleanup_list;	/* namespaces on death row */
 	struct list_head	exit_list;	/* Use only net_mutex */

include/net/netns/hash.h

@@ -1,21 +1,10 @@
 #ifndef __NET_NS_HASH_H__
 #define __NET_NS_HASH_H__
 
-#include <asm/cache.h>
-
-struct net;
+#include <net/net_namespace.h>
 
 static inline unsigned net_hash_mix(struct net *net)
 {
-#ifdef CONFIG_NET_NS
-	/*
-	 * shift this right to eliminate bits, that are
-	 * always zeroed
-	 */
-
-	return (unsigned)(((unsigned long)net) >> L1_CACHE_SHIFT);
-#else
-	return 0;
-#endif
+	return net->hash_mix;
 }
 #endif

net/core/net_namespace.c

@@ -153,6 +153,7 @@ static __net_init int setup_net(struct net *net, struct user_namespace *user_ns)
 
 	atomic_set(&net->count, 1);
 	atomic_set(&net->passive, 1);
+	get_random_bytes(&net->hash_mix, sizeof(u32));
 	net->dev_base_seq = 1;
 	net->user_ns = user_ns;