netdev - Re: [PATCH net] ipv6: make icmp6_send() robust against null skb->dev

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Message-ID: <CANn89i+_8QXCZfVPhCgKH5TNnKumhH=z5AAXepTfRDs1QwDDEA@mail.gmail.com>
Date:   Tue, 8 Jan 2019 03:14:28 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     p.sawicki2@...tner.samsung.com
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ipv6: make icmp6_send() robust against null skb->dev

On Tue, Jan 8, 2019 at 3:08 AM Piotr Sawicki
<p.sawicki2@...tner.samsung.com> wrote:

> Yes I know.  It looks like the Smack's security rule was changed during this process.
>
> Firstly the packet was allowed to be received and it was put into the backlog queue. Then, the
>
> rule was changed, and during the release phase LSM was called again for the same packet.
>
> But this time, Smack denied access and tried to send an ICMPv6 packet to inform a peer.
>
> I want to make sure if it is the root cause of this problem.
>
>
> Besides, what is the purpose of setting skb->dev to NULL in __sk_receive_skb() ?


We can not keep a pointer to the device, the device might be
dismantled/freed before socket backlog can be processed.

Input processing is using RCU, meaning no refcount is taken on the device.