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Subject: Re: [PATCH v2] net/smc: fix TCP fallback socket release
To:     Myungho Jung <mhjungk@gmail.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        linux-s390@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20181217052122.GA26299@myunghoj-Precision-5530>
From:   Ursula Braun <ubraun@linux.ibm.com>
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Date:   Mon, 17 Dec 2018 15:58:58 +0100
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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@syzkaller.appspotmail.com
> Reported-by: syzbot+e3132895630f957306bc@syzkaller.appspotmail.com
> Signed-off-by: Myungho Jung <mhjungk@gmail.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.

> +		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)
>