netdev - Re: net: use-after-free in neigh_timer_handler/sock

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Date:   Wed, 1 Mar 2017 13:54:26 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        James Morris <jmorris@...ei.org>
Subject: Re: net: use-after-free in neigh_timer_handler/sock_wfree

On Wed, Mar 1, 2017 at 1:43 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>>
>> This one looks very similar to a previous one:
>> https://groups.google.com/forum/#!topic/syzkaller/BhyN5OFd7sQ
>>
>> Both happen on raw v6 sockets.
>>
>> For me, it seems the sk refcnt is not correct, skb should still hold
>> a refcnt so it should not be freed before kfree_skb() in a timer
>> handler...
>
> More precisely, after this commit:
>
> commit 2b85a34e911bf483c27cfdd124aeb1605145dc80
> Author: Eric Dumazet <eric.dumazet@...il.com>
> Date:   Thu Jun 11 02:55:43 2009 -0700
>
>     net: No more expensive sock_hold()/sock_put() on each tx
>
> we don't take (old) refcnt any more on TX path, sk_wmem_alloc
> is the new refcnt. ;)

So the bug is that skb->truesize is mangled by reassembly unit,
while sbk->sk is tracking sk_wmem_alloc changes in order
to decide when it is safe to free sk.

This is why we need to call skb_orphan(), as we did for IPv4 in
8282f27449bf15548