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>

diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index df3eedb142ff809b9ce8f9dd49c41782f91aba42..4c0ad9b4dba011d90205454bed96571b3cb34b6c 100644 (file)
--- 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 */