linux-kernel - Re: [PATCH] rtc: wilco-ec: Sanitize values received from RTC

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Date:   Sun, 15 Sep 2019 23:44:03 +0100
From:   Nick Crews <ncrews@...omium.org>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>, linux-rtc@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Duncan Laurie <dlaurie@...gle.com>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>
Subject: Re: [PATCH] rtc: wilco-ec: Sanitize values received from RTC

Hi Alexandre, thanks for the thoughts.

On Thu, Sep 12, 2019 at 9:09 AM Alexandre Belloni
<alexandre.belloni@...tlin.com> wrote:
>
> Hi Nick,
>
> On 10/09/2019 16:19:29+0100, Nick Crews wrote:
> > Check that the time received from the RTC HW is valid,
> > otherwise the computation of rtc_year_days() in the next
> > line could, and sometimes does, crash the kernel.
> >
> > While we're at it, fix the license to plain "GPL".
> >
> > Signed-off-by: Nick Crews <ncrews@...omium.org>
> > ---
> >  drivers/rtc/rtc-wilco-ec.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/rtc/rtc-wilco-ec.c b/drivers/rtc/rtc-wilco-ec.c
> > index 8ad4c4e6d557..0ccbf2dce832 100644
> > --- a/drivers/rtc/rtc-wilco-ec.c
> > +++ b/drivers/rtc/rtc-wilco-ec.c
> > @@ -110,8 +110,16 @@ static int wilco_ec_rtc_read(struct device *dev, struct rtc_time *tm)
> >       tm->tm_mday     = rtc.day;
> >       tm->tm_mon      = rtc.month - 1;
> >       tm->tm_year     = rtc.year + (rtc.century * 100) - 1900;
> > -     tm->tm_yday     = rtc_year_days(tm->tm_mday, tm->tm_mon, tm->tm_year);
>
> If your driver doesn't care about yday, userspace doesn't either. You
> can simply not set it.

This driver indeed does not care about yday, so it sounds good to me
to simply not set it. However, I do still want to worry about the HW
returning some bogus time. It could be indicative of some other problem,
and I don't want to pass on this illegal time further up the stack. I can't
think of a reason why an illegal time should exist at all, we should just
deal with it as soon as possible. How about I remove setting yday as
you suggest, but still keep the rtc_valid_tm() check?

>
> >
> > +     if (rtc_valid_tm(tm)) {
> > +             dev_warn(dev,
> > +                      "Time computed from EC RTC is invalid: sec=%d, min=%d, hour=%d, mday=%d, mon=%d, year=%d",
> > +                      tm->tm_sec, tm->tm_min, tm->tm_hour, tm->mday,
> > +                      tm->mon, tm->year);
> > +             return -EIO;
> > +     }
> > +
> > +     tm->tm_yday = rtc_year_days(tm->tm_mday, tm->tm_mon, tm->tm_year);
> >       /* Don't compute day of week, we don't need it. */
> >       tm->tm_wday = -1;

Following our discussion, perhaps I'll just remove this too since the
RTC core inits this to -1 already?

> >
> > @@ -188,5 +196,5 @@ module_platform_driver(wilco_ec_rtc_driver);
> >
> >  MODULE_ALIAS("platform:rtc-wilco-ec");
> >  MODULE_AUTHOR("Nick Crews <ncrews@...omium.org>");
> > -MODULE_LICENSE("GPL v2");
> > +MODULE_LICENSE("GPL");
>
> This should be in a separate patch.

OK, I'll separate this out :)

>
> >  MODULE_DESCRIPTION("Wilco EC RTC driver");
> > --
> > 2.11.0
> >
>
> --
> Alexandre Belloni, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com