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

["Chatradhi, Naveen Krishna" <NaveenKrishna.Chatradhi@amd.com>]

[AMD Official Use Only - Approved for External Use]

Hi Guenter,

> 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:
Thanks for the tip, I will check this out.

> 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))
Do you mean if (time_after(jiffies, 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().

Regards,
Naveenk

-----Original Message-----
From: Guenter Roeck <linux@roeck-us.net> 
Sent: Sunday, November 22, 2020 7:00 PM
To: Chatradhi, Naveen Krishna <NaveenKrishna.Chatradhi@amd.com>
Cc: Salvatore Bonaccorso <carnil@debian.org>; linux-hwmon@vger.kernel.org; naveenkrishna.ch@gmail.com; stable@vger.kernel.org
Subject: Re: [PATCH] hwmon: amd_energy: modify the visibility of the counters

[CAUTION: External Email]

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