netdev - Re: [PATCH net] ppp: Fix a scheduling-while-atomic bug in del

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Date:   Tue, 1 Aug 2017 13:39:21 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Gao Feng <gfree.wind@....163.com>
Cc:     xeb@...l.ru, David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net] ppp: Fix a scheduling-while-atomic bug in del_chan

On Mon, Jul 31, 2017 at 3:07 AM,  <gfree.wind@....163.com> wrote:
> From: Gao Feng <gfree.wind@....163.com>
>
> The PPTP set the pptp_sock_destruct as the sock's sk_destruct, it would
> trigger this bug when __sk_free is invoked in atomic context, because of
> the call path pptp_sock_destruct->del_chan->synchronize_rcu.
>
> Now move the synchronize_rcu to pptp_release from del_chan. This is the
> only one case which would free the sock and need the synchronize_rcu.

I don't understand the last part.

>From my understanding, this RCU is supposed to protect the pppox_sock
pointers in 'callid_sock' which could be NULL'ed in del_chan(). And the
pppox_sock is freed when the last refcnt is gone, that is, when sock
dctor is called. pptp_release() is ONLY called when the fd in user-space
is gone, not necessarily the last refcnt.