netfilter: nf_conntrack: fix the hash random initializing race

nf_conntrack_alloc() isn't called with nf_conntrack_lock locked, so hash
random initializing code maybe executed more than once on different
CPUs.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

1 files changed, 13 insertions, 6 deletions

diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index df3eedb142f..4c0ad9b4dba 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -65,8 +65,7 @@ EXPORT_SYMBOL_GPL(nf_conntrack_max);
 DEFINE_PER_CPU(struct nf_conn, nf_conntrack_untracked);
 EXPORT_PER_CPU_SYMBOL(nf_conntrack_untracked);
 
-static int nf_conntrack_hash_rnd_initted;
-static unsigned int nf_conntrack_hash_rnd;
+static unsigned int nf_conntrack_hash_rnd __read_mostly;
 
 static u_int32_t __hash_conntrack(const struct nf_conntrack_tuple *tuple,
                                   u16 zone, unsigned int size, unsigned int rnd)
@@ -574,10 +573,18 @@ struct nf_conn *nf_conntrack_alloc(struct net *net, u16 zone,
 {
         struct nf_conn *ct;
 
-        if (unlikely(!nf_conntrack_hash_rnd_initted)) {
-                get_random_bytes(&nf_conntrack_hash_rnd,
-                                sizeof(nf_conntrack_hash_rnd));
-                nf_conntrack_hash_rnd_initted = 1;
+        if (unlikely(!nf_conntrack_hash_rnd)) {
+                unsigned int rand;
+
+                /*
+                 * Why not initialize nf_conntrack_rnd in a "init()" function ?
+                 * Because there isn't enough entropy when system initializing,
+                 * and we initialize it as late as possible.
+                 */
+                do {
+                        get_random_bytes(&rand, sizeof(rand));
+                } while (!rand);
+                cmpxchg(&nf_conntrack_hash_rnd, 0, rand);
         }
 
         /* We don't want any race condition at early drop stage */