[PATCH] [RFC] netfilter: nf_conntrack: don't relase a conntrack with non-zero refcnt

[Andrey Vagin <avagin@openvz.org>]

----
Eric and Florian, could you look at this patch. When you say,
that it looks good, I will ask the user to validate it.
I can't reorder these actions, because it's reproduced on a real host
with real users. Thanks.
----

nf_conntrack_free can't be called for a conntract with non-zero ref-counter,
because it can race with nf_conntrack_find_get().

A conntrack slab is created with SLAB_DESTROY_BY_RCU. Non-zero
ref-conunter says that this conntrack is used now. So when we release a
conntrack with non-zero counter, we break this assumption.

CPU1                                    CPU2
____nf_conntrack_find()
                                        nf_ct_put()
                                         destroy_conntrack()
                                        ...
                                        init_conntrack
                                         __nf_conntrack_alloc (set use = 1)
atomic_inc_not_zero(&ct->use) (use = 2)
                                         if (!l4proto->new(ct, skb, dataoff, timeouts))
                                          nf_conntrack_free(ct); (use = 2 !!!)
                                        ...
                                        __nf_conntrack_alloc (set use = 1)
 if (!nf_ct_key_equal(h, tuple, zone))
  nf_ct_put(ct); (use = 0)
   destroy_conntrack()
                                        /* continue to work with CT */

After applying the path "[PATCH] netfilter: nf_conntrack: fix RCU race
in nf_conntrack_find_get (v3)" another bug was triggered in
destroy_conntrack():
<4>[67096.759334] ------------[ cut here ]------------
<2>[67096.759353] kernel BUG at net/netfilter/nf_conntrack_core.c:211!
...
<4>[67096.759837] Pid: 498649, comm: atdd veid: 666 Tainted: G         C ---------------    2.6.32-042stab084.18 #1 042stab084_18 /DQ45CB
<4>[67096.759932] RIP: 0010:[<ffffffffa03d99ac>]  [<ffffffffa03d99ac>] destroy_conntrack+0x15c/0x190 [nf_conntrack]
<4>[67096.760255] Call Trace:
<4>[67096.760255]  [<ffffffff814844a7>] nf_conntrack_destroy+0x17/0x30
<4>[67096.760255]  [<ffffffffa03d9bb5>] nf_conntrack_find_get+0x85/0x130 [nf_conntrack]
<4>[67096.760255]  [<ffffffffa03d9fb2>] nf_conntrack_in+0x352/0xb60 [nf_conntrack]
<4>[67096.760255]  [<ffffffffa048c771>] ipv4_conntrack_local+0x51/0x60 [nf_conntrack_ipv4]
<4>[67096.760255]  [<ffffffff81484419>] nf_iterate+0x69/0xb0
<4>[67096.760255]  [<ffffffff814b5b00>] ? dst_output+0x0/0x20
<4>[67096.760255]  [<ffffffff814845d4>] nf_hook_slow+0x74/0x110
<4>[67096.760255]  [<ffffffff814b5b00>] ? dst_output+0x0/0x20
<4>[67096.760255]  [<ffffffff814b66d5>] raw_sendmsg+0x775/0x910
<4>[67096.760255]  [<ffffffff8104c5a8>] ? flush_tlb_others_ipi+0x128/0x130
<4>[67096.760255]  [<ffffffff8100bc4e>] ? apic_timer_interrupt+0xe/0x20
<4>[67096.760255]  [<ffffffff8100bc4e>] ? apic_timer_interrupt+0xe/0x20
<4>[67096.760255]  [<ffffffff814c136a>] inet_sendmsg+0x4a/0xb0
<4>[67096.760255]  [<ffffffff81444e93>] ? sock_sendmsg+0x13/0x140
<4>[67096.760255]  [<ffffffff81444f97>] sock_sendmsg+0x117/0x140
<4>[67096.760255]  [<ffffffff8102e299>] ? native_smp_send_reschedule+0x49/0x60
<4>[67096.760255]  [<ffffffff81519beb>] ? _spin_unlock_bh+0x1b/0x20
<4>[67096.760255]  [<ffffffff8109d930>] ? autoremove_wake_function+0x0/0x40
<4>[67096.760255]  [<ffffffff814960f0>] ? do_ip_setsockopt+0x90/0xd80
<4>[67096.760255]  [<ffffffff8100bc4e>] ? apic_timer_interrupt+0xe/0x20
<4>[67096.760255]  [<ffffffff8100bc4e>] ? apic_timer_interrupt+0xe/0x20
<4>[67096.760255]  [<ffffffff814457c9>] sys_sendto+0x139/0x190
<4>[67096.760255]  [<ffffffff810efa77>] ? audit_syscall_entry+0x1d7/0x200
<4>[67096.760255]  [<ffffffff810ef7c5>] ? __audit_syscall_exit+0x265/0x290
<4>[67096.760255]  [<ffffffff81474daf>] compat_sys_socketcall+0x13f/0x210
<4>[67096.760255]  [<ffffffff8104dea3>] ia32_sysret+0x0/0x5

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Vasiliy Averin <vvs@parallels.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
---
 include/net/netfilter/nf_conntrack.h |  1 -
 net/netfilter/nf_conntrack_core.c    | 18 +++++++++++-------
 net/netfilter/nf_conntrack_netlink.c |  2 +-
 net/netfilter/nf_synproxy_core.c     |  4 ++--
 net/netfilter/xt_CT.c                |  2 +-
 5 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/include/net/netfilter/nf_conntrack.h b/include/net/netfilter/nf_conntrack.h
index 01ea6ee..d338316 100644
--- a/include/net/netfilter/nf_conntrack.h
+++ b/include/net/netfilter/nf_conntrack.h
@@ -243,7 +243,6 @@ void nf_ct_untracked_status_or(unsigned long bits);
 void nf_ct_iterate_cleanup(struct net *net,
 			   int (*iter)(struct nf_conn *i, void *data),
 			   void *data, u32 portid, int report);
-void nf_conntrack_free(struct nf_conn *ct);
 struct nf_conn *nf_conntrack_alloc(struct net *net, u16 zone,
 				   const struct nf_conntrack_tuple *orig,
 				   const struct nf_conntrack_tuple *repl,
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index b56e53b..c38cc74 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -198,6 +198,8 @@ clean_from_lists(struct nf_conn *ct)
 	nf_ct_remove_expectations(ct);
 }
 
+static void nf_conntrack_free(struct nf_conn *ct);
+
 static void
 destroy_conntrack(struct nf_conntrack *nfct)
 {
@@ -226,9 +228,8 @@ destroy_conntrack(struct nf_conntrack *nfct)
 	 * too. */
 	nf_ct_remove_expectations(ct);
 
-	/* We overload first tuple to link into unconfirmed or dying list.*/
-	BUG_ON(hlist_nulls_unhashed(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode));
-	hlist_nulls_del_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode);
+	if (!hlist_nulls_unhashed(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode))
+		hlist_nulls_del_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode);
 
 	NF_CT_STAT_INC(net, delete);
 	spin_unlock_bh(&nf_conntrack_lock);
@@ -772,18 +773,21 @@ struct nf_conn *nf_conntrack_alloc(struct net *net, u16 zone,
 }
 EXPORT_SYMBOL_GPL(nf_conntrack_alloc);
 
-void nf_conntrack_free(struct nf_conn *ct)
+static void nf_conntrack_free(struct nf_conn *ct)
 {
 	struct net *net = nf_ct_net(ct);
 
+	/* A freed object has refcnt == 0, thats
+	 * the golden rule for SLAB_DESTROY_BY_RCU
+	 */
+	NF_CT_ASSERT(atomic_read(&ct->ct_general.use) == 0);
+
 	nf_ct_ext_destroy(ct);
 	nf_ct_ext_free(ct);
 	kmem_cache_free(net->ct.nf_conntrack_cachep, ct);
 	smp_mb__before_atomic_dec();
 	atomic_dec(&net->ct.count);
 }
-EXPORT_SYMBOL_GPL(nf_conntrack_free);
-
 
 /* Allocate a new conntrack: we return -ENOMEM if classification
    failed due to stress.  Otherwise it really is unclassifiable. */
@@ -835,7 +839,7 @@ init_conntrack(struct net *net, struct nf_conn *tmpl,
 	}
 
 	if (!l4proto->new(ct, skb, dataoff, timeouts)) {
-		nf_conntrack_free(ct);
+		nf_ct_put(ct);
 		pr_debug("init conntrack: can't track with proto module\n");
 		return NULL;
 	}
diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index 3e91ad3..fadd0f3 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -1732,7 +1732,7 @@ ctnetlink_create_conntrack(struct net *net, u16 zone,
 err2:
 	rcu_read_unlock();
 err1:
-	nf_conntrack_free(ct);
+	nf_ct_put(ct);
 	return ERR_PTR(err);
 }
 
diff --git a/net/netfilter/nf_synproxy_core.c b/net/netfilter/nf_synproxy_core.c
index 9858e3e..d12234c 100644
--- a/net/netfilter/nf_synproxy_core.c
+++ b/net/netfilter/nf_synproxy_core.c
@@ -381,7 +381,7 @@ static int __net_init synproxy_net_init(struct net *net)
 err3:
 	free_percpu(snet->stats);
 err2:
-	nf_conntrack_free(ct);
+	nf_ct_put(ct);
 err1:
 	return err;
 }
@@ -390,7 +390,7 @@ static void __net_exit synproxy_net_exit(struct net *net)
 {
 	struct synproxy_net *snet = synproxy_pernet(net);
 
-	nf_conntrack_free(snet->tmpl);
+	nf_ct_put(snet->tmpl);
 	synproxy_proc_exit(net);
 	free_percpu(snet->stats);
 }
diff --git a/net/netfilter/xt_CT.c b/net/netfilter/xt_CT.c
index da35ac0..da4edfe 100644
--- a/net/netfilter/xt_CT.c
+++ b/net/netfilter/xt_CT.c
@@ -237,7 +237,7 @@ out:
 	return 0;
 
 err3:
-	nf_conntrack_free(ct);
+	nf_ct_put(ct);
 err2:
 	nf_ct_l3proto_module_put(par->family);
 err1:
-- 
1.8.4.2