netdev - Re: Latest net-next from GIT panic

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Message-ID: <1505932231.29839.106.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Wed, 20 Sep 2017 11:30:31 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...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, 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.