netdev - Re: [PATCH net] ax25: fix possible use-after-free

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Date:   Wed, 23 Jan 2019 15:42:41 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ax25: fix possible use-after-free



On 01/23/2019 03:25 PM, Cong Wang wrote:
> On Tue, Jan 22, 2019 at 10:41 AM 'Eric Dumazet' via syzkaller
> <syzkaller@...glegroups.com> wrote:
>>
>> syzbot found that ax25 routes where not properly protected
>> against concurrent use [1].
>>
>> In this particular report the bug happened while
>> copying ax25->digipeat.
>>
>> Fix this problem by making sure we call ax25_get_route()
>> while ax25_route_lock is held, so that no modification
>> could happen while using the route.
> 
> ax25_route_lock_use() is a read lock, so two ax25_rt_autobind()
> could still enter the same critical section?
> 

Not sure I understand your concern.

The two ax25_rt_autobind() would only read the route contents,
so that should be fine ?

> 
>>
>> The current two ax25_get_route() callers do not sleep,
>> so this change should be fine.
>>
>> Once we do that, ax25_get_route() no longer needs to
>> grab a reference on the found route.
> .
> 
> After your patch, ax25_hold_route() has no callers while
> ax25_put_route() still does. Is ->refcount always 1?

Yes, the plan is to remove this refcount in net-next.