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

From: Srinivas Pandruvada <[email-masked]>
Date: Fri, 05 Jan 2018 17:53:55 -0800

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

> 
> Based on an original patch by Elena Reshetova.
> 
> Cc: Srinivas Pandruvada <[email-masked]>
> Cc: Zhang Rui <[email-masked]>
> Cc: Eduardo Valentin <[email-masked]>
> Signed-off-by: Elena Reshetova <[email-masked]>
> Signed-off-by: Dan Williams <[email-masked]>
> ---
>  .../thermal/int340x_thermal/int340x_thermal_zone.c |   14 ++++++++
> ------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/thermal/int340x_thermal/int340x_thermal_zone.c
> b/drivers/thermal/int340x_thermal/int340x_thermal_zone.c
> index 145a5c53ff5c..442a1d9bf7ad 100644
> --- a/drivers/thermal/int340x_thermal/int340x_thermal_zone.c
> +++ b/drivers/thermal/int340x_thermal/int340x_thermal_zone.c
> @@ -17,6 +17,7 @@
>  #include <linux/init.h>
>  #include <linux/acpi.h>
>  #include <linux/thermal.h>
> +#include <linux/compiler.h>
>  #include "int340x_thermal_zone.h"
>  
>  static int int340x_thermal_get_zone_temp(struct thermal_zone_device
> *zone,
> @@ -52,20 +53,21 @@ static int int340x_thermal_get_trip_temp(struct
> thermal_zone_device *zone,
>  					 int trip, int *temp)
>  {
>  	struct int34x_thermal_zone *d = zone->devdata;
> +	unsigned long *elem;
>  	int i;
>  
>  	if (d->override_ops && d->override_ops->get_trip_temp)
>  		return d->override_ops->get_trip_temp(zone, trip,
> temp);
>  
> -	if (trip < d->aux_trip_nr)
> -		*temp = d->aux_trips[trip];
> -	else if (trip == d->crt_trip_id)
> +	if ((elem = nospec_array_ptr(d->aux_trips, trip, d-
> >aux_trip_nr))) {
> +		*temp = *elem;
> +	} else if (trip == d->crt_trip_id) {
>  		*temp = d->crt_temp;
> -	else if (trip == d->psv_trip_id)
> +	} else if (trip == d->psv_trip_id) {
>  		*temp = d->psv_temp;
> -	else if (trip == d->hot_trip_id)
> +	} else if (trip == d->hot_trip_id) {
>  		*temp = d->hot_temp;
> -	else {
> +	} else {
>  		for (i = 0; i < INT340X_THERMAL_MAX_ACT_TRIP_COUNT;
> i++) {
>  			if (d->act_trips[i].valid &&
>  			    d->act_trips[i].id == trip) {
>