[PATCH net] pptp: dst_release sk_dst_cache in pptp_sock_destruct

[Xin Long <lucien.xin@gmail.com>]

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@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.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));
 }
 
 static int pptp_create(struct net *net, struct socket *sock, int kern)
-- 
2.1.0