netdev - Re: [PATCH net] ppp: Fix a scheduling-while-atomic bug in del

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Date:   Wed, 2 Aug 2017 10:13:36 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Gao Feng <gfree.wind@....163.com>
Cc:     xeb@...l.ru, David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net] ppp: Fix a scheduling-while-atomic bug in del_chan

Hi, Gao

On Tue, Aug 1, 2017 at 1:39 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> From my understanding, this RCU is supposed to protect the pppox_sock
> pointers in 'callid_sock' which could be NULL'ed in del_chan(). And the
> pppox_sock is freed when the last refcnt is gone, that is, when sock
> dctor is called. pptp_release() is ONLY called when the fd in user-space
> is gone, not necessarily the last refcnt.

Your commit is probably not the right fix. Can you try the following fix?

diff --git a/drivers/net/ppp/pptp.c b/drivers/net/ppp/pptp.c
index 6dde9a0cfe76..e75bb95c107f 100644
--- a/drivers/net/ppp/pptp.c
+++ b/drivers/net/ppp/pptp.c
@@ -519,7 +519,6 @@ static int pptp_release(struct socket *sock)

        po = pppox_sk(sk);
        del_chan(po);
-       synchronize_rcu();

        pppox_unbind_sock(sk);
        sk->sk_state = PPPOX_DEAD;
@@ -564,6 +563,7 @@ static int pptp_create(struct net *net, struct
socket *sock, int kern)
        sk->sk_family      = PF_PPPOX;
        sk->sk_protocol    = PX_PROTO_PPTP;
        sk->sk_destruct    = pptp_sock_destruct;
+       sock_set_flag(sk, SOCK_RCU_FREE);

        po = pppox_sk(sk);
        opt = &po->proto.pptp;