linux-kernel - Re: [PATCH v2] net/smc: fix TCP fallback socket release

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Message-ID: <20181218070339.GA20290@myunghoj-Precision-5530>
Date:   Mon, 17 Dec 2018 23:03:40 -0800
From:   Myungho Jung <mhjungk@...il.com>
To:     Ursula Braun <ubraun@...ux.ibm.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] net/smc: fix TCP fallback socket release

On Mon, Dec 17, 2018 at 03:58:58PM +0100, Ursula Braun wrote:
> 

Hi Ursula,

Thank you for your suggestion. I have a question on your comment.

> 
> On 12/17/2018 06:21 AM, Myungho Jung wrote:
> > clcsock can be released while kernel_accept() references it in TCP
> > listen worker. Also, clcsock needs to wake up before released if TCP
> > fallback is used and the clcsock is blocked by accept. Add a lock to
> > safely release clcsock and call kernel_sock_shutdown() to wake up
> > clcsock from accept in smc_release().
> 
> Thanks for your effort to solve this problem. I have some minor
> improvement proposals:
> 
> > 
> > Reported-by: syzbot+0bf2e01269f1274b4b03@...kaller.appspotmail.com
> > Reported-by: syzbot+e3132895630f957306bc@...kaller.appspotmail.com
> > Signed-off-by: Myungho Jung <mhjungk@...il.com>
> > ---
> >  net/smc/af_smc.c | 14 ++++++++++++--
> >  net/smc/smc.h    |  2 ++
> >  2 files changed, 14 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
> > index 5fbaf1901571..5d06fb1bbccf 100644
> > --- a/net/smc/af_smc.c
> > +++ b/net/smc/af_smc.c
> > @@ -147,8 +147,14 @@ static int smc_release(struct socket *sock)
> >  		sk->sk_shutdown |= SHUTDOWN_MASK;
> >  	}
> >  	if (smc->clcsock) {
> > +		if (smc->use_fallback && sk->sk_state == SMC_LISTEN) {
> > +			/* wake up clcsock accept */
> > +			rc = kernel_sock_shutdown(smc->clcsock, SHUT_RDWR);
> > +		}
> 
> This part is not needed, since an SMC socket in state SMC_LISTEN is never
> a use_fallback socket.

In smc_sendmsg(), set use_fallback to true if SMC socket is SMC_INIT
state and the message has MSG_FASTOPEN flag. After this, smc_listen()
would trigger smc_tcp_listen_work(). Is this not an expected scenario?
Then, what is the reason for not skipping smc_sendmsg() in SMC_INIT
state?

> 
> > +		mutex_lock(&smc->clcsock_release_lock);
> >  		sock_release(smc->clcsock);
> >  		smc->clcsock = NULL;
> > +		mutex_unlock(&smc->clcsock_release_lock);
> >  	}
> >  	if (smc->use_fallback) {
> >  		if (sk->sk_state != SMC_LISTEN && sk->sk_state != SMC_INIT)
> > @@ -205,6 +211,7 @@ static struct sock *smc_sock_alloc(struct net *net, struct socket *sock,
> >  	spin_lock_init(&smc->conn.send_lock);
> >  	sk->sk_prot->hash(sk);
> >  	sk_refcnt_debug_inc(sk);
> > +	mutex_init(&smc->clcsock_release_lock);
> >  
> >  	return sk;
> >  }
> > @@ -821,7 +828,7 @@ static int smc_clcsock_accept(struct smc_sock *lsmc, struct smc_sock **new_smc)
> >  	struct socket *new_clcsock = NULL;
> >  	struct sock *lsk = &lsmc->sk;
> >  	struct sock *new_sk;
> > -	int rc;
> > +	int rc = 0;
> 
> Without clcsock the good path should not be executed. Thus I suggest
> to initialize with something negative like -EINVAL.
> 
> >  
> >  	release_sock(lsk);
> >  	new_sk = smc_sock_alloc(sock_net(lsk), NULL, lsk->sk_protocol);
> > @@ -834,7 +841,10 @@ static int smc_clcsock_accept(struct smc_sock *lsmc, struct smc_sock **new_smc)
> >  	}
> >  	*new_smc = smc_sk(new_sk);
> >  
> > -	rc = kernel_accept(lsmc->clcsock, &new_clcsock, 0);
> > +	mutex_lock(&lsmc->clcsock_release_lock);
> > +	if (lsmc->clcsock)
> > +		rc = kernel_accept(lsmc->clcsock, &new_clcsock, 0);
> > +	mutex_unlock(&lsmc->clcsock_release_lock);
> >  	lock_sock(lsk);
> >  	if  (rc < 0)
> >  		lsk->sk_err = -rc;
> > diff --git a/net/smc/smc.h b/net/smc/smc.h
> > index 08786ace6010..9a2795cf5d30 100644
> > --- a/net/smc/smc.h
> > +++ b/net/smc/smc.h
> > @@ -219,6 +219,8 @@ struct smc_sock {				/* smc sock container */
> >  						 * started, waiting for unsent
> >  						 * data to be sent
> >  						 */
> > +	struct mutex            clcsock_release_lock;
> > +						/* protects clcsock */
> 
> I suggest to be more precise: "protects clcsock of a listen socket" 
> 
> >  };
> >  
> >  static inline struct smc_sock *smc_sk(const struct sock *sk)
> > 
>