Re: [Patch net] ppp: defer netns reference release for ppp channel

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Tue, Jul 05, 2016 at 10:12:36PM -0700, Cong Wang wrote:
> Matt reported that we have a NULL pointer dereference
> in ppp_pernet() from ppp_connect_channel(),
> i.e. pch->chan_net is NULL.
> 
> This is due to that a parallel ppp_unregister_channel()
> could happen while we are in ppp_connect_channel(), during
> which pch->chan_net set to NULL. Since we need a reference
> to net per channel, it makes sense to sync the refcnt
> with the life time of the channel, therefore we should
> release this reference when we destroy it.
> 
> Fixes: 1f461dcdd296 ("ppp: take reference on channels netns")
> Reported-by: Matt Bennett <Matt.Bennett@alliedtelesis.co.nz>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: linux-ppp@vger.kernel.org
> Cc: Guillaume Nault <g.nault@alphalink.fr>
> Cc: Cyrill Gorcunov <gorcunov@openvz.org>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> ---

Hi Cong! I may be wrong, but this doesn't look right in general.
We take the net in ppp_register_channel->ppp_register_net_channel
and (name) context implies that ppp_unregister_channel does
the reverse. Maybe there some sync point missed? I'll review
in detail a bit later.