netdev - Re: Latest net-next from GIT panic

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Message-ID: <CAM_iQpVF3Gf3G=543o8g+5N2dtEL4DcUPN3a6=L8V_G0CXZ65w@mail.gmail.com>
Date:   Wed, 20 Sep 2017 11:36:18 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Paweł Staszewski <pstaszewski@...are.pl>,
        Wei Wang <weiwan@...gle.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: Latest net-next from GIT panic

On Wed, Sep 20, 2017 at 11:30 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2017-09-20 at 11:22 -0700, Cong Wang wrote:
>> but dmesg at this time shows nothing about interfaces or flaps.
>>
>> This is very odd.
>>
>> We only free netdevice in free_netdev() and it is only called when
>> we unregister a netdevice. Otherwise pcpu_refcnt is impossible
>> to be NULL.
>
> If there is a missing dev_hold() or one dev_put() in excess,
> this would allow the netdev to be freed too soon.
>
> -> Use after free.
> memory holding netdev could be reallocated-cleared by some other kernel
> user.
>

Sure, but only unregister could trigger a free. If there is no unregister,
like what Pawel claims, then there is no free, the refcnt just goes to
0 but the memory is still there.