linux-kernel - Re: [PATCH v2 2/6] x86/sgx: Do not consider unsanitized pages an error

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Date:   Thu, 1 Sep 2022 10:50:07 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "jarkko@...nel.org" <jarkko@...nel.org>
CC:     "pmenzel@...gen.mpg.de" <pmenzel@...gen.mpg.de>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "Dhanraj, Vijay" <vijay.dhanraj@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "haitao.huang@...ux.intel.com" <haitao.huang@...ux.intel.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/6] x86/sgx: Do not consider unsanitized pages an
 error

On Wed, 2022-08-31 at 13:39 -0700, Reinette Chatre wrote:
> >   static int ksgxd(void *p)
> >   {
> > +	long ret;
> > +
> >   	set_freezable();
> >   
> >   	/*
> >   	 * Sanitize pages in order to recover from kexec(). The 2nd pass is
> >   	 * required for SECS pages, whose child pages blocked EREMOVE.
> >   	 */
> > -	__sgx_sanitize_pages(&sgx_dirty_page_list);
> > -	__sgx_sanitize_pages(&sgx_dirty_page_list);
> > +	ret = __sgx_sanitize_pages(&sgx_dirty_page_list);
> > +	if (ret == -ECANCELED)
> > +		/* kthread stopped */
> > +		return 0;
> >   
> > -	/* sanity check: */
> > -	WARN_ON(!list_empty(&sgx_dirty_page_list));
> > +	ret = __sgx_sanitize_pages(&sgx_dirty_page_list);
> > +	switch (ret) {
> > +	case 0:
> > +		/* success, no unsanitized pages */
> > +		break;
> > +
> > +	case -ECANCELED:
> > +		/* kthread stopped */
> > +		return 0;
> > +
> > +	default:
> > +		/*
> > +		 * Never expected to happen in a working driver. If it
> > happens
> > +		 * the bug is expected to be in the sanitization process,
> > but
> > +		 * successfully sanitized pages are still valid and driver
> > can
> > +		 * be used and most importantly debugged without issues. To
> > put
> > +		 * short, the global state of kernel is not corrupted so no
> > +		 * reason to do any more complicated rollback.
> > +		 */
> > +		pr_err("%ld unsanitized pages\n", ret);
> > +	}
> >   
> >   	while (!kthread_should_stop()) {
> >   		if (try_to_freeze())
> 
> 
> I think I am missing something here. A lot of logic is added here but I
> do not see why it is necessary.  ksgxd() knows via kthread_should_stop() if
> the reclaimer was canceled. I am thus wondering, could the above not be
> simplified to something similar to V1:
> 
> @@ -388,6 +393,8 @@ void sgx_reclaim_direct(void)
>  
>  static int ksgxd(void *p)
>  {
> +	unsigned long left_dirty;
> +
>  	set_freezable();
>  
>  	/*
> @@ -395,10 +402,10 @@ static int ksgxd(void *p)
>  	 * required for SECS pages, whose child pages blocked EREMOVE.
>  	 */
>  	__sgx_sanitize_pages(&sgx_dirty_page_list);
> -	__sgx_sanitize_pages(&sgx_dirty_page_list);
>  
> -	/* sanity check: */
> -	WARN_ON(!list_empty(&sgx_dirty_page_list));
> +	left_dirty = __sgx_sanitize_pages(&sgx_dirty_page_list);
> +	if (left_dirty && !kthread_should_stop())
> +		pr_err("%lu unsanitized pages\n", left_dirty);
>  

This basically means driver bug if I understand correctly.  To be consistent
with the behaviour of existing code, how about just WARN()?
	
	...
	left_dirty = __sgx_sanitize_pages(&sgx_dirty_page_list);
	WARN_ON(left_dirty && !kthread_should_stop());

It seems there's little value to print out the unsanitized pages here.  The
existing code doesn't print it anyway.

-- 
Thanks,
-Kai