Date:	Wed, 20 Jul 2016 16:59:36 +0000
From:	"Chen, Yu C" <yu.c.chen@...el.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Linux PM <linux-pm@...r.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH][v2] timekeeping: Fix memory overwrite of sleep_time_bin
 array

Hi,
> -----Original Message-----
> From: Rafael J. Wysocki [mailto:rjw@...ysocki.net]
> Sent: Wednesday, July 20, 2016 9:00 PM
> To: Chen, Yu C
> Cc: Thomas Gleixner; John Stultz; Linux PM; Linux Kernel Mailing List
> Subject: Re: [PATCH][v2] timekeeping: Fix memory overwrite of sleep_time_bin
> array
> 
> On Wednesday, July 20, 2016 07:06:58 PM Chen Yu wrote:
> > Hi Thomas,
> > On Tue, Jul 19, 2016 at 12:40:14PM +0200, Thomas Gleixner wrote:
> > > On Tue, 19 Jul 2016, Chen Yu wrote:
> > > > On 2016年07月19日 16:36, Thomas Gleixner wrote:
> > > > > On Tue, 19 Jul 2016, Chen Yu wrote:
> > > > > > Further investigation shows that, the problem is caused by
> > > > > > setting /sys/power/pm_trace to 1 before the 1st hibernation,
> > > > > > since once pm_trace is enabled, the rtc becomes an
> > > > > > unmeaningful value after resumed,
> > > > >
> > > > > So why is the RTC value useless if pm_trace is enabled? I really
> > > > > have a hard time to understand why pm_trace would affect the
> > > > > sleep time readout from RTC.
> > > >
> > > > After pm_trace is enabled, during system suspend/hibernate, the
> > > > hash name of each devices will be written to rtc, so the rtc value
> > > > depends on what we write in last suspend round, thus pm_trace can
> > > > be used for diagnose which device failed to suspend(eg, the
> > > > suspending on this device hang the system, we reboot the system , and
> check rtc hash value).
> > > >
> > > > In our case, after first hibernate/resume round, we found our
> > > > current system time is at 2117, so syscore_resume ->
> timekeeping_resume :
> > > > __timekeeping_inject_sleeptime(tk, &ts_delta) would inject a quite
> > > > large delta : 2117 - 2017 year, thus the sleep_time_bin is overflow.
> > >
> > > While the range check is certainly correct and a good thing to have
> > > it's wrong in the first place to call
> > > __timekeeping_inject_sleeptime() in case that pm_trace is enabled
> > > simply because that "hash" time value will also wreckage
> > > timekeeping. Your patch is just curing the symptom in the debug code but
> not fixing the root cause.
> > >
> > OK. I've modified the patch.
> > In case I break any other stuff :p, could you help check if this patch
> > is in the right direction, thanks:
> >
> > 1.  There are two places would invoke __timekeeping_inject_sleeptime(),
> >     they are timekeeping_resume and rtc_resume, so we need to deal with
> >     them respctively.
> >
> > 2.  for rtc_resume, if the pm_trace has once been enabled,
> >     we bypass the injection of sleep time.
> >
> > 3. for timekeeping_resume,
> >      Currently we either use nonstop clock source, or use persistent
> >      clock to get the sleep time. As pm_trace breaks systems who use rtc
> >      as a persistent clock, x86 is affected. So we add a
> >      check for x86 that, if the pm_trace has been enabled, we can not
> >      trust the persistent clock delta read from rtc, thus bypass
> >      the injection of sleep time in this case.
> >
> > 4. Why we checked the history of pm_trace: once pm_trace
> >    has been enabled, the delta of rtc would not be reliable anymore.
> >    For example, if we only check current pm_trace, we might still get
> >    memory overwrite:
> >
> >    4.1 echo 1 > /sys/power/pm_trace
> >    4.2 hibernate/resume (rtc is broken, do not add delta from rtc because
> pm_trace is 1)
> >    4.3 echo 0 > /sys/power/pm_trace
> >    4.4 hibernate/resume (rtc is still broken, but add delta from rtc
> > because pm_trace is 0)
> 
> The initial state of the RTC is invalid, but will the delta be still invalid?
>
According to feedback from the bug reporter, 
with previous patch applied which uses pm_trace_is_enabled() directly in
 resume, then after several rounds,  once the pm_trace is disabled, 
the following hibernate/resume  would bring  a overflow sleep delta.
 It looks like the delta also considers the historical(previous) delta, 
so I added the history pm_once_trace . I'll confirm and give feedback later.
 
> And what if the admin fixes up the RTC before hibernating?  You will still
> discard the RTC delta until the next reboot, right?
Yes, it will be discarded, I agree we should not bypass the delta if someone has
fixed the rtc, I'll dig into the code.
 
Thanks,
Yu

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