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

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Message-ID: <CAM_iQpWmJmk=_yoV8=krOWonE_0orTC5Z4GaCnPg8Jv3GX6dmw@mail.gmail.com>
Date:   Mon, 7 Aug 2017 10:17:02 -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: Re: [PATCH net] ppp: Fix a scheduling-while-atomic bug in del_chan

On Sun, Aug 6, 2017 at 6:32 PM, Gao Feng <gfree.wind@....163.com> wrote:
> I think the RCU should be supposed to avoid the race between del_chan and lookup_chan.

More precisely, it is callid_sock which is protected by RCU.

Unless I miss any other code path, pptp_exit_module() is
problematic too, I don't think it can just vfree() the whole thing.

> The synchronize_rcu could make sure if there was one which calls lookup_chan in this period, it would be finished and the sock refcnt is increased if necessary.
>
> So I think it is ok to invoke sock_put directly without SOCK_RCU_FREE, because the lookup_chan caller has already hold the sock refcnt,
>

If you mean the sock_hold() inside lookup_chan(), no,
it doesn't help because we already dereference the sock
before it.

Also, lookup_chan_dst() does not have a refcnt, I don't
find any code preventing it deref'ing other sock in callid_sock
than the calling one.