netdev - Re: [PATCH net] pptp: dst_release sk_dst_cache in pptp_sock

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Date:   Fri, 8 Mar 2019 09:17:24 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Xin Long <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>
Cc:     davem@...emloft.net, Dmitry Kozlov <xeb@...l.ru>
Subject: Re: [PATCH net] pptp: dst_release sk_dst_cache in pptp_sock_destruct



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)

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