On Fri, 9 Apr 2021 10:48:52 -0700, Guenter Roeck wrote:
> Commit 60268b0e8258 ("hwmon: (amd_energy) modify the visibility of
> the counters") restricted visibility of AMD energy counters to work
> around a side-channel attack using energy data to determine which
> instructions are executed. The attack is described in 'PLATYPUS:
> Software-based Power Side-Channel Attacks on x86'. It relies on quick
> and accurate energy readings.
>
> Limiting energy readings to privileged users is annoying. A much better
> solution is to make energy readings unusable for attacks by randomizing
> the time between updates. We can do that by caching energy values for
> a short and randomized period of time.
>
> Cc: Naveen Krishna Chatradhi <nchatrad@xxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> ---
> v2: Simplified code by using unified function to accumulate energy data
>
> drivers/hwmon/amd_energy.c | 29 +++++++++++++++++++++--------
> 1 file changed, 21 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/hwmon/amd_energy.c b/drivers/hwmon/amd_energy.c
> index 93bad64039f1..1bf0afc2740c 100644
> --- a/drivers/hwmon/amd_energy.c
> +++ b/drivers/hwmon/amd_energy.c
> @@ -18,6 +18,7 @@
> #include <linux/mutex.h>
> #include <linux/processor.h>
> #include <linux/platform_device.h>
> +#include <linux/random.h>
> #include <linux/sched.h>
> #include <linux/slab.h>
> #include <linux/topology.h>
> @@ -35,6 +36,7 @@
> struct sensor_accumulator {
> u64 energy_ctr;
> u64 prev_value;
> + unsigned long cache_timeout;
> };
>
> struct amd_energy_data {
> @@ -74,17 +76,14 @@ static void get_energy_units(struct amd_energy_data *data)
> data->energy_units = (rapl_units & AMD_ENERGY_UNIT_MASK) >> 8;
> }
>
Add a comment stating that this function must be called with accum's
&data->lock held?
> -static void accumulate_delta(struct amd_energy_data *data,
> - int channel, int cpu, u32 reg)
> +static void __accumulate_delta(struct sensor_accumulator *accum,
> + int cpu, u32 reg)
> {
> - struct sensor_accumulator *accum;
> u64 input;
>
> - mutex_lock(&data->lock);
> rdmsrl_safe_on_cpu(cpu, reg, &input);
> input &= AMD_ENERGY_MASK;
>
> - accum = &data->accums[channel];
> if (input >= accum->prev_value)
> accum->energy_ctr +=
> input - accum->prev_value;
> @@ -93,6 +92,14 @@ static void accumulate_delta(struct amd_energy_data *data,
> accum->prev_value + input;
>
> accum->prev_value = input;
> + accum->cache_timeout = jiffies + HZ + get_random_int() % HZ;
Needs #include <linux/jiffies.h> maybe?
> +}
> +
> +static void accumulate_delta(struct amd_energy_data *data,
> + int channel, int cpu, u32 reg)
> +{
> + mutex_lock(&data->lock);
> + __accumulate_delta(&data->accums[channel], cpu, reg);
> mutex_unlock(&data->lock);
> }
>
> @@ -124,6 +131,7 @@ static int amd_energy_read(struct device *dev,
> {
> struct amd_energy_data *data = dev_get_drvdata(dev);
> struct sensor_accumulator *accum;
> + u64 energy;
> u32 reg;
> int cpu;
>
> @@ -140,10 +148,15 @@ static int amd_energy_read(struct device *dev,
> reg = ENERGY_CORE_MSR;
> }
>
> - accumulate_delta(data, channel, cpu, reg);
> accum = &data->accums[channel];
>
> - *val = div64_ul(accum->energy_ctr * 1000000UL, BIT(data->energy_units));
> + mutex_lock(&data->lock);
> + if (!accum->energy_ctr || time_after(jiffies, accum->cache_timeout))
> + __accumulate_delta(accum, cpu, reg);
> + energy = accum->energy_ctr;
> + mutex_unlock(&data->lock);
> +
> + *val = div64_ul(energy * 1000000UL, BIT(data->energy_units));
>
> return 0;
> }
> @@ -152,7 +165,7 @@ static umode_t amd_energy_is_visible(const void *_data,
> enum hwmon_sensor_types type,
> u32 attr, int channel)
> {
> - return 0440;
> + return 0444;
> }
>
> static int energy_accumulator(void *p)
Very nice. This will make the driver useful again :-)
Reviewed-by: Jean Delvare <jdelvare@xxxxxxx>
--
Jean Delvare
SUSE L3 Support
