netfilter: nf_conntrack: don't release a conntrack with non-zero refcnt

With this patch, the conntrack refcount is initially set to zero and
it is bumped once it is added to any of the list, so we fulfill
Eric's golden rule which is that all released objects always have a
refcount that equals zero.

Andrey Vagin reports that nf_conntrack_free can't be called for a
conntrack 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-counter says that this conntrack is used. 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" 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

I have reused the original title for the RFC patch that Andrey posted and
most of the original patch description.

Cc: Eric Dumazet <[e-mail]>
Cc: Andrew Vagin <[e-mail]>
Cc: Florian Westphal <[e-mail]>
Reported-by: Andrew Vagin <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Acked-by: Andrew Vagin <[e-mail]>

include/net/netfilter/nf_conntrack.h

···
 extern unsigned int nf_conntrack_hash_rnd;
 void init_nf_conntrack_hash_rnd(void);
 
+void nf_conntrack_tmpl_insert(struct net *net, struct nf_conn *tmpl);
+
 #define NF_CT_STAT_INC(net, count)	  __this_cpu_inc((net)->ct.stat->count)
 #define NF_CT_STAT_INC_ATOMIC(net, count) this_cpu_inc((net)->ct.stat->count)
 

net/netfilter/nf_conntrack_core.c

···
 			goto out;
 
 	add_timer(&ct->timeout);
-	nf_conntrack_get(&ct->ct_general);
+	smp_wmb();
+	/* The caller holds a reference to this object */
+	atomic_set(&ct->ct_general.use, 2);
 	__nf_conntrack_hash_insert(ct, hash, repl_hash);
 	NF_CT_STAT_INC(net, insert);
 	spin_unlock_bh(&nf_conntrack_lock);
···
 	return -EEXIST;
 }
 EXPORT_SYMBOL_GPL(nf_conntrack_hash_check_insert);
+
+/* deletion from this larval template list happens via nf_ct_put() */
+void nf_conntrack_tmpl_insert(struct net *net, struct nf_conn *tmpl)
+{
+	__set_bit(IPS_TEMPLATE_BIT, &tmpl->status);
+	__set_bit(IPS_CONFIRMED_BIT, &tmpl->status);
+	nf_conntrack_get(&tmpl->ct_general);
+
+	spin_lock_bh(&nf_conntrack_lock);
+	/* Overload tuple linked list to put us in template list. */
+	hlist_nulls_add_head_rcu(&tmpl->tuplehash[IP_CT_DIR_ORIGINAL].hnnode,
+				 &net->ct.tmpl);
+	spin_unlock_bh(&nf_conntrack_lock);
+}
+EXPORT_SYMBOL_GPL(nf_conntrack_tmpl_insert);
 
 /* Confirm a connection given skb; places it in hash table */
 int
···
 		nf_ct_zone->id = zone;
 	}
 #endif
-	/*
-	 * changes to lookup keys must be done before setting refcnt to 1
+	/* Because we use RCU lookups, we set ct_general.use to zero before
+	 * this is inserted in any list.
 	 */
-	smp_wmb();
-	atomic_set(&ct->ct_general.use, 1);
+	atomic_set(&ct->ct_general.use, 0);
 	return ct;
 
 #ifdef CONFIG_NF_CONNTRACK_ZONES
···
 void nf_conntrack_free(struct nf_conn *ct)
 {
 	struct net *net = nf_ct_net(ct);
+
+	/* A freed object has refcnt == 0, that's
+	 * 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);
···
 		__nf_ct_try_assign_helper(ct, tmpl, GFP_ATOMIC);
 		NF_CT_STAT_INC(net, new);
 	}
+
+	/* Now it is inserted into the unconfirmed list, bump refcount */
+	nf_conntrack_get(&ct->ct_general);
 
 	/* Overload tuple linked list to put us in unconfirmed list. */
 	hlist_nulls_add_head_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode,

net/netfilter/nf_synproxy_core.c

···
 		goto err2;
 	if (!nfct_synproxy_ext_add(ct))
 		goto err2;
-	__set_bit(IPS_TEMPLATE_BIT, &ct->status);
-	__set_bit(IPS_CONFIRMED_BIT, &ct->status);
 
+	nf_conntrack_tmpl_insert(net, ct);
 	snet->tmpl = ct;
 
 	snet->stats = alloc_percpu(struct synproxy_stats);
···
 {
 	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);
 }

net/netfilter/xt_CT.c

···
 			goto err3;
 	}
 
-	__set_bit(IPS_TEMPLATE_BIT, &ct->status);
-	__set_bit(IPS_CONFIRMED_BIT, &ct->status);
-
-	/* Overload tuple linked list to put us in template list. */
-	hlist_nulls_add_head_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode,
-				 &par->net->ct.tmpl);
+	nf_conntrack_tmpl_insert(par->net, ct);
 out:
 	info->ct = ct;
 	return 0;