[PATCH 4.9 60/76] netns: provide pure entropy for net_hash_mix()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit 355b98553789b646ed97ad801a619ff898471b92 ]

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: 0b4419162aa6 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 include/net/net_namespace.h |    1 +
 include/net/netns/hash.h    |   15 ++-------------
 net/core/net_namespace.c    |    1 +
 3 files changed, 4 insertions(+), 13 deletions(-)

--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -53,6 +53,7 @@ struct net {
 						 */
 	spinlock_t		rules_mod_lock;
 
+	u32			hash_mix;
 	atomic64_t		cookie_gen;
 
 	struct list_head	list;		/* list of network namespaces */
--- a/include/net/netns/hash.h
+++ b/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 u32 net_hash_mix(const struct net *net)
 {
-#ifdef CONFIG_NET_NS
-	/*
-	 * shift this right to eliminate bits, that are
-	 * always zeroed
-	 */
-
-	return (u32)(((unsigned long)net) >> L1_CACHE_SHIFT);
-#else
-	return 0;
-#endif
+	return net->hash_mix;
 }
 #endif
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -282,6 +282,7 @@ static __net_init int setup_net(struct n
 
 	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;
 	idr_init(&net->netns_ids);