Re: [PATCH] hwmon: amd_energy: modify the visibility of the counters

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

On Sun, Nov 22, 2020 at 06:56:24AM +0000, Chatradhi, Naveen Krishna wrote:
> [AMD Official Use Only - Approved for External Use]
> 
> Hi Guenter, Salvatore
> 
> > This is very unusual, and may mess up the "sensors" command.
> > What problem is this trying to solve ?
> Guenter, sorry for the delayed response.
> This fix is required to address the possible side channel attack reported in CVE-2020-12912.
> 
[ ... ]
> 
> >> ?
> Yes, Salvatore, thanks for bringing the links. 
> 
A much better fix would have been to cache RAPL data for a short period
of time. To avoid any possibility of attacks, maybe add some random
interval. Something like this:

In accumulate_delta():
	accums->next_update = jiffies + HZ / 2 + get_random_int % HZ;

In amd_energy_read():
	accum = &data->accums[channel];
	if (time_after(accum->next_update))
		accumulate_delta(data, channel, cpu, reg);
	*val = div64_ul(accum->energy_ctr * 1000000UL, BIT(data->energy_units));

and drop amd_add_delta().

Guenter