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Date:   Tue, 12 Mar 2019 18:05:17 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, davem <davem@...emloft.net>,
        Dmitry Kozlov <xeb@...l.ru>
Subject: Re: [PATCH net] pptp: dst_release sk_dst_cache in pptp_sock_destruct

On Sat, Mar 9, 2019 at 1:17 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 03/07/2019 11:25 PM, Xin Long wrote:
> > sk_setup_caps() is called to set sk->sk_dst_cache in pptp_connect,
> > so we have to dst_release(sk->sk_dst_cache) in pptp_sock_destruct,
> > otherwise, the dst refcnt will leak.
> >
> > It can be reproduced by this syz log:
> >
> >   r1 = socket$pptp(0x18, 0x1, 0x2)
> >   bind$pptp(r1, &(0x7f0000000100)={0x18, 0x2, {0x0, @local}}, 0x1e)
> >   connect$pptp(r1, &(0x7f0000000000)={0x18, 0x2, {0x3, @remote}}, 0x1e)
> >
> > Consecutive dmesg warnings will occur:
> >
> >   unregister_netdevice: waiting for lo to become free. Usage count = 1
> >
> > Fixes: 00959ade36ac ("PPTP: PPP over IPv4 (Point-to-Point Tunneling Protocol)")
> > Reported-by: Xiumei Mu <xmu@...hat.com>
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > ---
> >  drivers/net/ppp/pptp.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/net/ppp/pptp.c b/drivers/net/ppp/pptp.c
> > index 8f09edd..76172c2 100644
> > --- a/drivers/net/ppp/pptp.c
> > +++ b/drivers/net/ppp/pptp.c
> > @@ -532,6 +532,7 @@ static void pptp_sock_destruct(struct sock *sk)
> >               pppox_unbind_sock(sk);
> >       }
> >       skb_queue_purge(&sk->sk_receive_queue);
> > +     dst_release(rcu_dereference_check(sk->sk_dst_cache, 1));
>
> rcu_dereference_protected() is probably more appropriate. (No barrier is needed)
Seems so, the update is prevented then.
I will post v2. Thanks.

>
> >  }
> >
> >  static int pptp_create(struct net *net, struct socket *sock, int kern)
> >

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