Patch "netfilter: nf_ct_ext: fix possible panic after nf_ct_extend_unregister" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

netfilter: nf_ct_ext: fix possible panic after nf_ct_extend_unregister

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
netfilter-nf_ct_ext-fix-possible-panic-after-nf_ct_extend_unregister.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Liping Zhang <[email protected]>
Date: Sat, 25 Mar 2017 16:35:29 +0800
Subject: netfilter: nf_ct_ext: fix possible panic after nf_ct_extend_unregister

From: Liping Zhang <[email protected]>

commit 9c3f3794926a997b1cab6c42480ff300efa2d162 upstream.

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.

---
 net/netfilter/nf_conntrack_extend.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

@@ -53,7 +53,11 @@ nf_ct_ext_create(struct nf_ct_ext **ext,
 
 	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_c
 
 	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_c
 	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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/netfilter-nf_ct_ext-fix-possible-panic-after-nf_ct_extend_unregister.patch