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

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Message-ID: <CAM_iQpXoPGFj=ey7_aYKhhfPdL1SMxQj=adydxVYqwPJhBOSpg@mail.gmail.com>
Date:   Wed, 23 Jan 2019 15:25:53 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ax25: fix possible use-after-free

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?


>
> 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?

Thanks.