Commit 60268b0e8258 ("hwmon: (amd_energy) modify the visibility of
the counters") restricted visibility of AMD energy counters to work
around a side-channel attack. The attack is described in 'PLATYPUS:
Software-based Power Side-Channel Attacks on x86'. It relies on quick
and accurate power readings.Limiting power readings to privileged users is annoying. A much better solution is to make power 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 <[email protected]> Cc: Greg Kroah-Hartman <[email protected]> Fixes: 60268b0e8258 ("hwmon: (amd_energy) modify the visibility of the counters") Signed-off-by: Guenter Roeck <[email protected]> --- drivers/hwmon/amd_energy.c | 28 +++++++++++++++++++--------- 1 file changed, 19 insertions(+), 9 deletions(-) diff --git a/drivers/hwmon/amd_energy.c b/drivers/hwmon/amd_energy.c index a86cc8d6d93d..2ee6919eea57 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 +36,8 @@ struct sensor_accumulator { u64 energy_ctr; u64 prev_value; + u64 cached_value; + unsigned long cache_timeout; }; struct amd_energy_data { @@ -93,6 +96,8 @@ static void accumulate_delta(struct amd_energy_data *data, accum->prev_value + input; accum->prev_value = input; + accum->cached_value = input; + accum->cache_timeout = jiffies + HZ + get_random_int() % HZ; mutex_unlock(&data->lock); } @@ -125,16 +130,21 @@ static void amd_add_delta(struct amd_energy_data *data, int ch, u64 input; mutex_lock(&data->lock); - rdmsrl_safe_on_cpu(cpu, reg, &input); - input &= AMD_ENERGY_MASK; accum = &data->accums[ch]; - if (input >= accum->prev_value) - input += accum->energy_ctr - - accum->prev_value; - else - input += UINT_MAX - accum->prev_value + - accum->energy_ctr; + if (!accum->cached_value || time_after(jiffies, accum->cache_timeout)) { + rdmsrl_safe_on_cpu(cpu, reg, &input); + input &= AMD_ENERGY_MASK; + + if (input >= accum->prev_value) + input += accum->energy_ctr - accum->prev_value; + else + input += UINT_MAX - accum->prev_value + accum->energy_ctr; + accum->cached_value = input; + accum->cache_timeout = jiffies + HZ + get_random_int() % HZ; + } else { + input = accum->cached_value; + } /* Energy consumed = (1/(2^ESU) * RAW * 1000000UL) μJoules */ *val = div64_ul(input * 1000000UL, BIT(data->energy_units)); @@ -171,7 +181,7 @@ 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
