Subject: [PATCH 13/38] pptp: dst_release sk_dst_cache in pptp_sock_destruct

From: Xin Long <[email-masked]>
Date: Wed, 13 Mar 2019 17:00:48 +0800

[ Upstream commit 9417d81f4f8adfe20a12dd1fadf73a618cbd945d ]

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

v1->v2:
  - use rcu_dereference_protected() instead of rcu_dereference_check(),
    as suggested by Eric.

Fixes: 00959ade36ac ("PPTP: PPP over IPv4 (Point-to-Point Tunneling Protocol)")
Reported-by: Xiumei Mu <[email-masked]>
Signed-off-by: Xin Long <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 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 67ffe74747a1..7321a4eca235 100644
--- a/drivers/net/ppp/pptp.c
+++ b/drivers/net/ppp/pptp.c
@@ -537,6 +537,7 @@ static void pptp_sock_destruct(struct sock *sk)
 		pppox_unbind_sock(sk);
 	}
 	skb_queue_purge(&sk->sk_receive_queue);
+	dst_release(rcu_dereference_protected(sk->sk_dst_cache, 1));
 }
 
 static int pptp_create(struct net *net, struct socket *sock, int kern)
-- 
2.20.1