linux-kernel - Re: [PATCH] hwmon: (xgene) restrict power measurements to admin by default

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Message-ID: <86leud917u.fsf@scott-ph-mail.amperecomputing.com>
Date:   Fri, 03 Jun 2022 09:11:01 -0700
From:   D Scott Phillips <scott@...amperecomputing.com>
To:     Guenter Roeck <linux@...ck-us.net>, linux-hwmon@...r.kernel.org
Cc:     Jean Delvare <jdelvare@...e.com>, linux-kernel@...r.kernel.org,
        patches@...erecomputing.com,
        Darren Hart <darren@...amperecomputing.com>,
        Ilkka Koskinen <ilkka@...amperecomputing.com>
Subject: Re: [PATCH] hwmon: (xgene) restrict power measurements to admin by
 default

Guenter Roeck <linux@...ck-us.net> writes:

> On 6/3/22 08:21, D Scott Phillips wrote:
>> Access to power information can be used to infer the instructions being run
>> and possibly even data being processed on a cpu[1]. Restrict access to
>> power information to administrator users by default. (Cf. a similar
>> powercap change[2].)
>> 
>> [1]: Lipp, Moritz, et al. "PLATYPUS: software-based power side-channel
>>       attacks on x86." 2021 IEEE Symposium on Security and Privacy (SP).
>>       IEEE, 2021.
>> [2]: commit 949dd0104c49 ("powercap: restrict energy meter to root access")
>> 
>> Fixes: ed42cfa881e1 ("hwmon: Add xgene hwmon driver")
>> Signed-off-by: D Scott Phillips <scott@...amperecomputing.com>
>> Cc: stable@...r.kernel.org
>> ---
>>   drivers/hwmon/xgene-hwmon.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/hwmon/xgene-hwmon.c b/drivers/hwmon/xgene-hwmon.c
>> index 5cde837bfd09..6ad1daf2d354 100644
>> --- a/drivers/hwmon/xgene-hwmon.c
>> +++ b/drivers/hwmon/xgene-hwmon.c
>> @@ -397,9 +397,9 @@ static DEVICE_ATTR_RO(temp1_label);
>>   static DEVICE_ATTR_RO(temp1_input);
>>   static DEVICE_ATTR_RO(temp1_critical_alarm);
>>   static DEVICE_ATTR_RO(power1_label);
>> -static DEVICE_ATTR_RO(power1_input);
>> +static DEVICE_ATTR_ADMIN_RO(power1_input);
>>   static DEVICE_ATTR_RO(power2_label);
>> -static DEVICE_ATTR_RO(power2_input);
>> +static DEVICE_ATTR_ADMIN_RO(power2_input);
>>   
>>   static struct attribute *xgene_hwmon_attrs[] = {
>>   	&dev_attr_temp1_label.attr,
>
>
> NACK. The hwmon ABI expects world read access. Either find a workaround
> by making the measurements less accurate, or drop the driver. Also see
> commit 9049572fb145 ("hwmon: Remove amd_energy driver"), "Attribute access"
> in Documentation/hwmon/sysfs-interface.rst, and [1].

Ah, I see, thanks for pointing that out, I had missed that. I'm not
confident that the randomization approach can provide robustness against
this attack, so I'll follow up with a patch to drop the driver. Cheers,

Scott