linux-kernel - Re: net: use-after-free in neigh_timer_handler/sock

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Message-ID: <CAM_iQpVb62N=0veNSvCfgE7RfGcR=EKOgCtyF3c9t5b+Rk_avg@mail.gmail.com>
Date:   Wed, 1 Mar 2017 15:09:12 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <edumazet@...gle.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 1:54 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> 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.

That is my suspicion as well, skb->truesize is updated somewhere
but sk->sk_wmem_alloc isn't, so leads to this bug.

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


But I doubt skb_orphan() is the solution here, shouldn't we just
update sk->sk_wmem_alloc with skb->truesize changes?