[PATCH 3.16 129/129] netns: provide pure entropy for net_hash_mix()

[Ben Hutchings]

3.16.70-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <eduma...@google.com>

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: 0b4419162aa6 ("netns: introduce the net_hash_mix "salt" for hashes")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: Amit Klein <aksecur...@gmail.com>
Reported-by: Benny Pinkas <be...@pinkas.net>
Cc: Pavel Emelyanov <xe...@openvz.org>
Signed-off-by: David S. Miller <da...@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -54,6 +54,8 @@ 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 */
--- 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>
+#include <net/net_namespace.h>
 
-struct net;
-
-static inline unsigned int net_hash_mix(struct net *net)
+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 (unsigned)(((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
@@ -156,6 +156,7 @@ 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;