netfilter: nf_conntrack: fix confirmation race condition

New connection tracking entries are inserted into the hash before they
are fully set up, namely the CONFIRMED bit is not set and the timer not
started yet. This can theoretically lead to a race with timer, which
would set the timeout value to a relative value, most likely already in
the past.

Perform hash insertion as the final step to fix this.

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 5276a2dd56fe3b1d3d141970a882a27bdda7e968..b0b06c7a948315a69fb302e6052b41fcdf7bca84 100644 (file)
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -425,7 +425,6 @@ __nf_conntrack_confirm(struct sk_buff *skb)
        /* Remove from unconfirmed list */
        hlist_nulls_del_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode);
 
-       __nf_conntrack_hash_insert(ct, hash, repl_hash);
        /* Timer relative to confirmation time, not original
           setting time, otherwise we'd get timer wrap in
           weird delay cases. */
@@ -433,8 +432,16 @@ __nf_conntrack_confirm(struct sk_buff *skb)
        add_timer(&ct->timeout);
        atomic_inc(&ct->ct_general.use);
        set_bit(IPS_CONFIRMED_BIT, &ct->status);
+
+       /* Since the lookup is lockless, hash insertion must be done after
+        * starting the timer and setting the CONFIRMED bit. The RCU barriers
+        * guarantee that no other CPU can find the conntrack before the above
+        * stores are visible.
+        */
+       __nf_conntrack_hash_insert(ct, hash, repl_hash);
        NF_CT_STAT_INC(net, insert);
        spin_unlock_bh(&nf_conntrack_lock);
+
        help = nfct_help(ct);
        if (help && help->helper)
                nf_conntrack_event_cache(IPCT_HELPER, ct);