Subject: Re: [PATCH 12/18] Thermal/int340x: prevent bounds-check bypass via speculative execution

From: Srinivas Pandruvada <[email-masked]>
Date: Sat, 06 Jan 2018 09:24:35 -0800

On Fri, 2018-01-05 at 17:57 -0800, Dan Williams wrote:
> On Fri, Jan 5, 2018 at 5:53 PM, Srinivas Pandruvada
> <[email-masked]> wrote:
> > 
> > On Fri, 2018-01-05 at 17:10 -0800, Dan Williams wrote:
> > > 
> > > Static analysis reports that 'trip' may be a user controlled
> > > value
> > > that
> > > is used as a data dependency to read '*temp' from the 'd-
> > > >aux_trips'
> > > array.  In order to avoid potential leaks of kernel memory
> > > values,
> > > block
> > > speculative execution of the instruction stream that could issue
> > > reads
> > > based on an invalid value of '*temp'.
> > Not against the change as this is in a very slow path. But the trip
> > is
> > not an arbitrary value which user can enter.
> > 
> > This trip value is the one of the sysfs attribute in thermal zone.
> > For
> > example
> > 
> > # cd /sys/class/thermal/thermal_zone1
> > # ls trip_point_?_temp
> > trip_point_0_temp  trip_point_1_temp  trip_point_2_temp  trip_point
> > _3_t
> > emp  trip_point_4_temp  trip_point_5_temp  trip_point_6_temp
> > 
> > Here the "trip" is one of the above trip_point_*_temp. So in this
> > case
> > it can be from 0 to 6 as user can't do
> > # cat trip_point_7_temp
> > as there is no sysfs attribute for trip_point_7_temp.
> > 
> > The actual "trip" was obtained in thermal core via
> > 
> >       if (sscanf(attr->attr.name, "trip_point_%d_temp", &trip) !=
> > 1)
> >                 return -EINVAL;
> > 
> > Thanks,
> > Srinivas
> Ah, great, thanks. So do we even need the bounds check at that point?
We are not bound checking but the way we identify type of the trip.
Based on ACPI support we order trips:
- Aux trips max_count = aux_trip_nr
- One Critical trip
- One Hot trip
- One passive trip
- Rest all trips are active trips

So in the above example if d->aux_trip_nr is 1 then trip_point_0_temp
read/write is for aux trip. If d->aux_trip_nr is 0 then it can be any
other non aux trip.

BUT I am not still up to date with these attacks. Not sure about the
perimeter of user controlled value. It is a user controlled but limited
by the sysfs attributes. So I will test this patch and let you know if
there are any issues.

Thanks,
Srinivas