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

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Message-ID: <CAM_iQpVW3qR3XyytQX5aSbU4CYfCB5N8g2syyDfz1NSdWG7ihg@mail.gmail.com>
Date:   Wed, 23 Jan 2019 17:12:00 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "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 Wed, Jan 23, 2019 at 3:42 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> 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 ?

Not sure if it is read-only and safe for the following code:

        if (ax25_rt->digipeat != NULL) {
                ax25->digipeat = kmemdup(ax25_rt->digipeat, sizeof(ax25_digi),
                                         GFP_ATOMIC);
                if (ax25->digipeat == NULL) {
                        err = -ENOMEM;
                        goto put;
                }
                ax25_adjust_path(addr, ax25->digipeat);
        }

Maybe we leak memory here at least, not sure...


>
> >
> >>
> >> 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.
>

Good to know.

Thanks.