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 <redacted>
Cc: Paul Mackerras <redacted>
Cc: linux-ppp@vger.kernel.org
Cc: Guillaume Nault <redacted>
Cc: Cyrill Gorcunov <redacted>
Signed-off-by: Cong Wang <redacted>
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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.