linux-kernel - Re: [PATCH 1/6] x86/sgx: Do not consider unsanitized pages an error

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Message-ID: <Yw7AXxMFIEEU7TWA@kernel.org>
Date:   Wed, 31 Aug 2022 04:58:55 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     linux-sgx@...r.kernel.org,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        Vijay Dhanraj <vijay.dhanraj@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] x86/sgx: Do not consider unsanitized pages an error

On Wed, Aug 31, 2022 at 04:55:24AM +0300, Jarkko Sakkinen wrote:
> On Tue, Aug 30, 2022 at 03:54:27PM -0700, Reinette Chatre wrote:
> > Hi Jarkko,
> > 
> > On 8/29/2022 8:12 PM, Jarkko Sakkinen wrote:
> > > In sgx_init(), if misc_register() for the provision device fails, and
> > > neither sgx_drv_init() nor sgx_vepc_init() succeeds, then ksgxd will be
> > > prematurely stopped.
> > 
> > I do not think misc_register() is required to fail for the scenario to
> > be triggered (rather use "or" than "and"?). Perhaps just
> > "In sgx_init(), if a failure is encountered after ksgxd is started
> > (via sgx_page_reclaimer_init()) ...".
> 
> This would be the fixed version of the sentence:
> 
> "
> In sgx_init(), if misc_register() fails or misc_register() succeeds but
> neither sgx_drv_init() nor sgx_vepc_init() succeeds, then ksgxd will be
> prematurely stopped. This may leave some unsanitized pages, which does
> not matter, because SGX will be disabled for the whole power cycle.
> "
> 
> I want to keep the end states listed and not make it more abstract.
> 
> The second sentence addresses the remark below.
> 
> > To help the reader understand the subject of this patch it may help
> > to explain that prematurely stopping ksgxd may leave some
> > unsanitized pages, but that is not a problem since SGX cannot
> > be used on the platform anyway. 
> > 
> > > This triggers WARN_ON() because sgx_dirty_page_list ends up being
> > > non-empty, and dumps the call stack:
> > > 
> > 
> > Traces like below can be frowned upon. I recommend that you follow the
> > guidance in "Backtraces in commit mesages"(sic) in 
> > Documentation/process/submitting-patches.rst.
> > 
> > > [    0.268592] WARNING: CPU: 6 PID: 83 at
> > > arch/x86/kernel/cpu/sgx/main.c:401 ksgxd+0x1b7/0x1d0
> 
> Is this good enough? I had not actually spotted this section before but
> nice that it exists. Apparently has been added in 5.12.
> 
> >> > 
> > > Ultimately this can crash the kernel, if the following is set:
> > > 
> > > 	/proc/sys/kernel/panic_on_warn
> > > 
> > > Print a simple warning instead, and improve the output by printing the
> > > number of unsanitized pages, in order to provide debug informnation for
> > > future needs.
> > 
> > informnation -> information
> 
> +1
> 
> > 
> >  
> > ...
> > 
> > > Link: https://lore.kernel.org/linux-sgx/20220825051827.246698-1-jarkko@kernel.org/T/#u
> > > Reported-by: Paul Menzel <pmenzel@...gen.mpg.de>
> > > Tested-by: Paul Menzel <pmenzel@...gen.mpg.de>
> > > Fixes: 51ab30eb2ad4 ("x86/sgx: Replace section->init_laundry_list with sgx_dirty_page_list")
> > > Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
> > 
> > Should this go to stable?
> 
> I guess it should. The hard reason for this that it can panic
> the kernel.
> 
> > 
> > > 
> > > diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> > > index 515e2a5f25bb..903100fcfce3 100644
> > > --- a/arch/x86/kernel/cpu/sgx/main.c
> > > +++ b/arch/x86/kernel/cpu/sgx/main.c
> > > @@ -49,17 +49,20 @@ static LIST_HEAD(sgx_dirty_page_list);
> > >   * Reset post-kexec EPC pages to the uninitialized state. The pages are removed
> > >   * from the input list, and made available for the page allocator. SECS pages
> > >   * prepending their children in the input list are left intact.
> > > + *
> > > + * Contents of the @dirty_page_list must be thread-local, i.e.
> > > + * not shared by multiple threads.
> > 
> > Did you intend to mention something about the needed locking here? It looks
> > like some information is lost during the move to the function description.
> 
> Nothing about the locking that concerns the parameter, as the
> sentence defines clear constraints for the caller.
> 
> > 
> > >   */
> > > -static void __sgx_sanitize_pages(struct list_head *dirty_page_list)
> > > +static int __sgx_sanitize_pages(struct list_head *dirty_page_list)
> > >  {
> > >  	struct sgx_epc_page *page;
> > > +	int left_dirty = 0;
> > 
> > I do not know how many pages this code should be ready for but at least
> > this could handle more by being an unsigned int considering that it is
> > always positive ... maybe even unsigned long?
> 
> I would go for 'long'. More information below.
> 
> > 
> > >  	LIST_HEAD(dirty);
> > >  	int ret;
> > >  
> > > -	/* dirty_page_list is thread-local, no need for a lock: */
> > >  	while (!list_empty(dirty_page_list)) {
> > >  		if (kthread_should_stop())
> > > -			return;
> > > +			break;
> > >  
> > >  		page = list_first_entry(dirty_page_list, struct sgx_epc_page, list);
> > >  
> > > @@ -92,12 +95,14 @@ static void __sgx_sanitize_pages(struct list_head *dirty_page_list)
> > >  		} else {
> > >  			/* The page is not yet clean - move to the dirty list. */
> > >  			list_move_tail(&page->list, &dirty);
> > > +			left_dirty++;
> > >  		}
> > >  
> > >  		cond_resched();
> > >  	}
> > >  
> > >  	list_splice(&dirty, dirty_page_list);
> > > +	return left_dirty;
> > >  }
> > >  
> > >  static bool sgx_reclaimer_age(struct sgx_epc_page *epc_page)
> > > @@ -388,6 +393,8 @@ void sgx_reclaim_direct(void)
> > >  
> > >  static int ksgxd(void *p)
> > >  {
> > > +	int 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)
> > > +		pr_warn("%d unsanitized pages\n", left_dirty);
> > >  
> > >  	while (!kthread_should_stop()) {
> > >  		if (try_to_freeze())
> > 
> > 
> > Reinette
> 
> We need to return -ECANCELED on premature stop, and number of
> pages otherwise.
> 
> In premature stop, nothing should be printed, as the number
> is by practical means a random number. Otherwise, it is an
> indicator of a bug in the driver, and therefore a non-zero
> number should be printed pr_err(), if that happens after the
> second call.

I.e. even though we print less we get more *information* what
is going inside the kernel. Warning is not correct for either
path IMHO.

BR, Jarkko