Re: [PATCH v2 2/2] hwmon: (amd_energy) Restore visibility of energy counters

[Guenter Roeck <linux@roeck-us.net>]

On 4/12/21 5:27 AM, Jean Delvare wrote:
> 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@amd.com>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
>> ---
>> 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@suse.de>
> 
I made the suggested changes.

Thanks a lot for the review!

Guenter