From: Liping Zhang <zlpnob...@gmail.com>
If one cpu is doing nf_ct_extend_unregister while another cpu is doing
__nf_ct_ext_add_length, then we may hit BUG_ON(t == NULL). Moreover,
there's no synchronize_rcu invocation after set nf_ct_ext_types[id] to
NULL, so it's possible that we may access invalid pointer.
But actually, most of the ct extends are built-in, so the problem listed
above will not happen. However, there are two exceptions: NF_CT_EXT_NAT
and NF_CT_EXT_SYNPROXY.
For _EXT_NAT, the panic will not happen, since adding the nat extend and
unregistering the nat extend are located in the same file(nf_nat_core.c),
this means that after the nat module is removed, we cannot add the nat
extend too.
For _EXT_SYNPROXY, synproxy extend may be added by init_conntrack, while
synproxy extend unregister will be done by synproxy_core_exit. So after
nf_synproxy_core.ko is removed, we may still try to add the synproxy
extend, then kernel panic may happen.
I know it's very hard to reproduce this issue, but I can play a tricky
game to make it happen very easily :)
Step 1. Enable SYNPROXY for tcp dport 1234 at FORWARD hook:
# iptables -I FORWARD -p tcp --dport 1234 -j SYNPROXY
Step 2. Queue the syn packet to the userspace at raw table OUTPUT hook.
Also note, in the userspace we only add a 20s' delay, then
reinject the syn packet to the kernel:
# iptables -t raw -I OUTPUT -p tcp --syn -j NFQUEUE --queue-num 1
Step 3. Using "nc 2.2.2.2 1234" to connect the server.
Step 4. Now remove the nf_synproxy_core.ko quickly:
# iptables -F FORWARD
# rmmod ipt_SYNPROXY
# rmmod nf_synproxy_core
Step 5. After 20s' delay, the syn packet is reinjected to the kernel.
Now you will see the panic like this:
kernel BUG at net/netfilter/nf_conntrack_extend.c:91!
Call Trace:
? __nf_ct_ext_add_length+0x53/0x3c0 [nf_conntrack]
init_conntrack+0x12b/0x600 [nf_conntrack]
nf_conntrack_in+0x4cc/0x580 [nf_conntrack]
ipv4_conntrack_local+0x48/0x50 [nf_conntrack_ipv4]
nf_reinject+0x104/0x270
nfqnl_recv_verdict+0x3e1/0x5f9 [nfnetlink_queue]
? nfqnl_recv_verdict+0x5/0x5f9 [nfnetlink_queue]
? nla_parse+0xa0/0x100
nfnetlink_rcv_msg+0x175/0x6a9 [nfnetlink]
[...]
One possible solution is to make NF_CT_EXT_SYNPROXY extend built-in, i.e.
introduce nf_conntrack_synproxy.c and only do ct extend register and
unregister in it, similar to nf_conntrack_timeout.c.
But having such a obscure restriction of nf_ct_extend_unregister is not a
good idea, so we should invoke synchronize_rcu after set nf_ct_ext_types
to NULL, and check the NULL pointer when do __nf_ct_ext_add_length. Then
it will be easier if we add new ct extend in the future.
Last, we use kfree_rcu to free nf_ct_ext, so rcu_barrier() is unnecessary
anymore, remove it too.
Signed-off-by: Liping Zhang <zlpnob...@gmail.com>
Acked-by: Florian Westphal <f...@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pa...@netfilter.org>
---
net/netfilter/nf_conntrack_extend.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff --git a/net/netfilter/nf_conntrack_extend.c
b/net/netfilter/nf_conntrack_extend.c
index 02bcf00c2492..008299b7f78f 100644
--- a/net/netfilter/nf_conntrack_extend.c
+++ b/net/netfilter/nf_conntrack_extend.c
@@ -53,7 +53,11 @@ nf_ct_ext_create(struct nf_ct_ext **ext, enum nf_ct_ext_id
id,
rcu_read_lock();
t = rcu_dereference(nf_ct_ext_types[id]);
- BUG_ON(t == NULL);
+ if (!t) {
+ rcu_read_unlock();
+ return NULL;
+ }
+
off = ALIGN(sizeof(struct nf_ct_ext), t->align);
len = off + t->len + var_alloc_len;
alloc_size = t->alloc_size + var_alloc_len;
@@ -88,7 +92,10 @@ void *__nf_ct_ext_add_length(struct nf_conn *ct, enum
nf_ct_ext_id id,
rcu_read_lock();
t = rcu_dereference(nf_ct_ext_types[id]);
- BUG_ON(t == NULL);
+ if (!t) {
+ rcu_read_unlock();
+ return NULL;
+ }
newoff = ALIGN(old->len, t->align);
newlen = newoff + t->len + var_alloc_len;
@@ -175,6 +182,6 @@ void nf_ct_extend_unregister(struct nf_ct_ext_type *type)
RCU_INIT_POINTER(nf_ct_ext_types[type->id], NULL);
update_alloc_size(type);
mutex_unlock(&nf_ct_ext_type_mutex);
- rcu_barrier(); /* Wait for completion of call_rcu()'s */
+ synchronize_rcu();
}
EXPORT_SYMBOL_GPL(nf_ct_extend_unregister);
--
2.1.4
