[PATCH net] netns: provide pure entropy for net_hash_mix()

[Eric Dumazet]

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>
---
 include/net/net_namespace.h |  1 +
 include/net/netns/hash.h    | 10 ++--------
 net/core/net_namespace.c    |  1 +
 3 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index 
a68ced28d8f47e09c6f6beabc56a22a04ae3f163..12689ddfc24c44fe3297d1eda548811d8061670b
 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -59,6 +59,7 @@ struct net {
                                                 */
        spinlock_t              rules_mod_lock;
 
+       u32                     hash_mix;
        atomic64_t              cookie_gen;
 
        struct list_head        list;           /* list of network namespaces */
diff --git a/include/net/netns/hash.h b/include/net/netns/hash.h
index 
16a842456189f2fc1a3363685b5dd4310a32b2b8..d9b665151f3d9e916f35620141542a5a145e6123
 100644
--- a/include/net/netns/hash.h
+++ b/include/net/netns/hash.h
@@ -2,16 +2,10 @@
 #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
-       return (u32)(((unsigned long)net) >> ilog2(sizeof(*net)));
-#else
-       return 0;
-#endif
+       return net->hash_mix;
 }
 #endif
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 
17f36317363d19dcdeb6a6e75a116220b078c2b0..7e6dcc6257011d8b60e132e97a0db229c39d1daf
 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -304,6 +304,7 @@ static __net_init int setup_net(struct net *net, struct 
user_namespace *user_ns)
 
        refcount_set(&net->count, 1);
        refcount_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);
-- 
2.21.0.392.gf8f6787159e-goog