[PATCH 3.2 67/74] netfilter: nf_conntrack: fix RCU race in nf_conntrack_find_get

From: Ben Hutchings
Date: Mon Oct 09 2017 - 09:51:21 EST

3.2.94-rc1 review patch. If anyone has any objections, please let me know.

------------------

From: Andrey Vagin <avagin@xxxxxxxxxx>

commit c6825c0976fa7893692e0e43b09740b419b23c09 upstream.

Lets look at destroy_conntrack:

hlist_nulls_del_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode);
...
nf_conntrack_free(ct)
kmem_cache_free(net->ct.nf_conntrack_cachep, ct);

net->ct.nf_conntrack_cachep is created with SLAB_DESTROY_BY_RCU.

The hash is protected by rcu, so readers look up conntracks without
locks.
A conntrack is removed from the hash, but in this moment a few readers
still can use the conntrack. Then this conntrack is released and another
thread creates conntrack with the same address and the equal tuple.
After this a reader starts to validate the conntrack:
* It's not dying, because a new conntrack was created
* nf_ct_tuple_equal() returns true.

But this conntrack is not initialized yet, so it can not be used by two
threads concurrently. In this case BUG_ON may be triggered from
nf_nat_setup_info().

Florian Westphal suggested to check the confirm bit too. I think it's
right.

task 1 task 2 task 3
nf_conntrack_find_get
____nf_conntrack_find
destroy_conntrack
hlist_nulls_del_rcu
nf_conntrack_free
kmem_cache_free
__nf_conntrack_alloc
kmem_cache_alloc
memset(&ct->tuplehash[IP_CT_DIR_MAX],
if (nf_ct_is_dying(ct))
if (!nf_ct_tuple_equal()

I'm not sure, that I have ever seen this race condition in a real life.
Currently we are investigating a bug, which is reproduced on a few nodes.
In our case one conntrack is initialized from a few tasks concurrently,
we don't have any other explanation for this.

<2>[46267.083061] kernel BUG at net/ipv4/netfilter/nf_nat_core.c:322!
...
<4>[46267.083951] RIP: 0010:[<ffffffffa01e00a4>] [<ffffffffa01e00a4>] nf_nat_setup_info+0x564/0x590 [nf_nat]
...
<4>[46267.085549] Call Trace:
<4>[46267.085622] [<ffffffffa023421b>] alloc_null_binding+0x5b/0xa0 [iptable_nat]
<4>[46267.085697] [<ffffffffa02342bc>] nf_nat_rule_find+0x5c/0x80 [iptable_nat]
<4>[46267.085770] [<ffffffffa0234521>] nf_nat_fn+0x111/0x260 [iptable_nat]
<4>[46267.085843] [<ffffffffa0234798>] nf_nat_out+0x48/0xd0 [iptable_nat]
<4>[46267.085919] [<ffffffff814841b9>] nf_iterate+0x69/0xb0
<4>[46267.085991] [<ffffffff81494e70>] ? ip_finish_output+0x0/0x2f0
<4>[46267.086063] [<ffffffff81484374>] nf_hook_slow+0x74/0x110
<4>[46267.086133] [<ffffffff81494e70>] ? ip_finish_output+0x0/0x2f0
<4>[46267.086207] [<ffffffff814b5890>] ? dst_output+0x0/0x20
<4>[46267.086277] [<ffffffff81495204>] ip_output+0xa4/0xc0
<4>[46267.086346] [<ffffffff814b65a4>] raw_sendmsg+0x8b4/0x910
<4>[46267.086419] [<ffffffff814c10fa>] inet_sendmsg+0x4a/0xb0
<4>[46267.086491] [<ffffffff814459aa>] ? sock_update_classid+0x3a/0x50
<4>[46267.086562] [<ffffffff81444d67>] sock_sendmsg+0x117/0x140
<4>[46267.086638] [<ffffffff8151997b>] ? _spin_unlock_bh+0x1b/0x20
<4>[46267.086712] [<ffffffff8109d370>] ? autoremove_wake_function+0x0/0x40
<4>[46267.086785] [<ffffffff81495e80>] ? do_ip_setsockopt+0x90/0xd80
<4>[46267.086858] [<ffffffff8100be0e>] ? call_function_interrupt+0xe/0x20
<4>[46267.086936] [<ffffffff8118cb10>] ? ub_slab_ptr+0x20/0x90
<4>[46267.087006] [<ffffffff8118cb10>] ? ub_slab_ptr+0x20/0x90
<4>[46267.087081] [<ffffffff8118f2e8>] ? kmem_cache_alloc+0xd8/0x1e0
<4>[46267.087151] [<ffffffff81445599>] sys_sendto+0x139/0x190
<4>[46267.087229] [<ffffffff81448c0d>] ? sock_setsockopt+0x16d/0x6f0
<4>[46267.087303] [<ffffffff810efa47>] ? audit_syscall_entry+0x1d7/0x200
<4>[46267.087378] [<ffffffff810ef795>] ? __audit_syscall_exit+0x265/0x290
<4>[46267.087454] [<ffffffff81474885>] ? compat_sys_setsockopt+0x75/0x210
<4>[46267.087531] [<ffffffff81474b5f>] compat_sys_socketcall+0x13f/0x210
<4>[46267.087607] [<ffffffff8104dea3>] ia32_sysret+0x0/0x5
<4>[46267.087676] Code: 91 20 e2 01 75 29 48 89 de 4c 89 f7 e8 56 fa ff ff 85 c0 0f 84 68 fc ff ff 0f b6 4d c6 41 8b 45 00 e9 4d fb ff ff e8 7c 19 e9 e0 <0f> 0b eb fe f6 05 17 91 20 e2 80 74 ce 80 3d 5f 2e 00 00 00 74
<1>[46267.088023] RIP [<ffffffffa01e00a4>] nf_nat_setup_info+0x564/0x590

Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Cc: Florian Westphal <fw@xxxxxxxxx>
Cc: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Cc: Patrick McHardy <kaber@xxxxxxxxx>
Cc: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Signed-off-by: Andrey Vagin <avagin@xxxxxxxxxx>
Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
net/netfilter/nf_conntrack_core.c | 21 +++++++++++++++++----
1 file changed, 17 insertions(+), 4 deletions(-)

--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -307,6 +307,21 @@ static void death_by_timeout(unsigned lo
nf_ct_put(ct);
}

+static inline bool
+nf_ct_key_equal(struct nf_conntrack_tuple_hash *h,
+ const struct nf_conntrack_tuple *tuple,
+ u16 zone)
+{
+ struct nf_conn *ct = nf_ct_tuplehash_to_ctrack(h);
+
+ /* A conntrack can be recreated with the equal tuple,
+ * so we need to check that the conntrack is confirmed
+ */
+ return nf_ct_tuple_equal(tuple, &h->tuple) &&
+ nf_ct_zone(ct) == zone &&
+ nf_ct_is_confirmed(ct);
+}
+
/*
* Warning :
* - Caller must take a reference on returned object
@@ -328,8 +343,7 @@ ____nf_conntrack_find(struct net *net, u
local_bh_disable();
begin:
hlist_nulls_for_each_entry_rcu(h, n, &net->ct.hash[bucket], hnnode) {
- if (nf_ct_tuple_equal(tuple, &h->tuple) &&
- nf_ct_zone(nf_ct_tuplehash_to_ctrack(h)) == zone) {
+ if (nf_ct_key_equal(h, tuple, zone)) {
NF_CT_STAT_INC(net, found);
local_bh_enable();
return h;
@@ -376,8 +390,7 @@ begin:
!atomic_inc_not_zero(&ct->ct_general.use)))
h = NULL;
else {
- if (unlikely(!nf_ct_tuple_equal(tuple, &h->tuple) ||
- nf_ct_zone(ct) != zone)) {
+ if (unlikely(!nf_ct_key_equal(h, tuple, zone))) {
nf_ct_put(ct);
goto begin;
}