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

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

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;
 }
 
-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;
+}
+
+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)
-- 
2.17.1