netdev - Re: [PATCH net] net/smc: Avoid overwriting the copies of clcsock callback functions

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Date:   Thu, 10 Feb 2022 10:50:06 +0800
From:   Tony Lu <tonylu@...ux.alibaba.com>
To:     Wen Gu <guwen@...ux.alibaba.com>
Cc:     kgraul@...ux.ibm.com, davem@...emloft.net, kuba@...nel.org,
        linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net/smc: Avoid overwriting the copies of clcsock
 callback functions

On Wed, Feb 09, 2022 at 10:10:53PM +0800, Wen Gu wrote:
> The callback functions of clcsock will be saved and replaced during
> the fallback. But if the fallback happens more than once, then the
> copies of these callback functions will be overwritten incorrectly,
> resulting in a loop call issue:
> 
> clcsk->sk_error_report
>  |- smc_fback_error_report() <------------------------------|
>      |- smc_fback_forward_wakeup()                          | (loop)
>          |- clcsock_callback()  (incorrectly overwritten)   |
>              |- smc->clcsk_error_report() ------------------|
> 
> So this patch fixes the issue by saving these function pointers only
> once in the fallback and avoiding overwriting.
> 
> Reported-by: syzbot+4de3c0e8a263e1e499bc@...kaller.appspotmail.com
> Fixes: 341adeec9ada ("net/smc: Forward wakeup to smc socket waitqueue after fallback")
> Link: https://lore.kernel.org/r/0000000000006d045e05d78776f6@google.com
> Signed-off-by: Wen Gu <guwen@...ux.alibaba.com>
> ---
>  net/smc/af_smc.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
> index 8c89d0b..306d9e8c 100644
> --- a/net/smc/af_smc.c
> +++ b/net/smc/af_smc.c
> @@ -667,14 +667,17 @@ static void smc_fback_error_report(struct sock *clcsk)
>  static int smc_switch_to_fallback(struct smc_sock *smc, int reason_code)
>  {
>  	struct sock *clcsk;
> +	int rc = 0;
>  
>  	mutex_lock(&smc->clcsock_release_lock);
>  	if (!smc->clcsock) {
> -		mutex_unlock(&smc->clcsock_release_lock);
> -		return -EBADF;
> +		rc = -EBADF;
> +		goto out;
>  	}
>  	clcsk = smc->clcsock->sk;
>  
> +	if (smc->use_fallback)
> +		goto out;
>  	smc->use_fallback = true;

I am wondering that there is a potential racing. If ->use_fallback is
setted to true, but the rest of replacing process is on the way, others
who tested and passed ->use_fallback, they would get old value before
replacing.

Thanks,
Tony Lu