Re: Problem: BUG_ON hit in ppp_pernet() when re-connect after changing shared key on LAC

[Cong Wang <xiyou.wangcong@gmail.com>]

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On Tue, Jul 5, 2016 at 10:59 AM, Cong Wang <xiyou.wangcong@gmail.com> wrote:
> On Mon, Jul 4, 2016 at 7:50 PM, Matt Bennett
> <Matt.Bennett@alliedtelesis.co.nz> wrote:
>> Using printk I have confirmed that ppp_pernet() is called from
>> ppp_connect_channel() when the BUG occurs (i.e. pch->chan_net is NULL).
>>
>> This behavior appears to have been introduced in commit 1f461dc ("ppp:
>> take reference on channels netns").
>
> We have some race condition here, where a parallel ppp_unregister_channel()
> could happen while we are in ppp_connect_channel().
>
> We need some synchronization for them. I am not sure what is the right lock
> here since ppp locking looks crazy.

Matt, could you try if the attached patch helps?

Thanks!

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diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 8dedafa..07f0e49 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -2601,8 +2601,6 @@ ppp_unregister_channel(struct ppp_channel *chan)
 	spin_lock_bh(&pn->all_channels_lock);
 	list_del(&pch->list);
 	spin_unlock_bh(&pn->all_channels_lock);
-	put_net(pch->chan_net);
-	pch->chan_net = NULL;
 
 	pch->file.dead = 1;
 	wake_up_interruptible(&pch->file.rwait);
@@ -3136,6 +3134,11 @@ ppp_disconnect_channel(struct channel *pch)
  */
 static void ppp_destroy_channel(struct channel *pch)
 {
+	if (pch->chan_net) {
+		put_net(pch->chan_net);
+		pch->chan_net = NULL;
+	}
+
 	atomic_dec(&channel_count);
 
 	if (!pch->file.dead) {