Subject: Re: [PATCH v4 8/9] drivers/perf: open access for CAP_SYS_PERFMON privileged process

From: Alexey Budankov <[email-masked]>
Date: Sat, 18 Jan 2020 21:48:24 +0300

On 17.01.2020 13:51, Will Deacon wrote:
> On Wed, Dec 18, 2019 at 12:30:29PM +0300, Alexey Budankov wrote:
>>
>> Open access to monitoring for CAP_SYS_PERFMON privileged processes.
>> For backward compatibility reasons access to the monitoring remains open
>> for CAP_SYS_ADMIN privileged processes but CAP_SYS_ADMIN usage for secure
>> monitoring is discouraged with respect to CAP_SYS_PERFMON capability.
>>
>> Signed-off-by: Alexey Budankov <[email-masked]>
>> ---
>>  drivers/perf/arm_spe_pmu.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
>> index 4e4984a55cd1..5dff81bc3324 100644
>> --- a/drivers/perf/arm_spe_pmu.c
>> +++ b/drivers/perf/arm_spe_pmu.c
>> @@ -274,7 +274,7 @@ static u64 arm_spe_event_to_pmscr(struct perf_event *event)
>>  	if (!attr->exclude_kernel)
>>  		reg |= BIT(SYS_PMSCR_EL1_E1SPE_SHIFT);
>>  
>> -	if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR) && capable(CAP_SYS_ADMIN))
>> +	if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR) && perfmon_capable())
>>  		reg |= BIT(SYS_PMSCR_EL1_CX_SHIFT);
>>  
>>  	return reg;
>> @@ -700,7 +700,7 @@ static int arm_spe_pmu_event_init(struct perf_event *event)
>>  		return -EOPNOTSUPP;
>>  
>>  	reg = arm_spe_event_to_pmscr(event);
>> -	if (!capable(CAP_SYS_ADMIN) &&
>> +	if (!perfmon_capable() &&
>>  	    (reg & (BIT(SYS_PMSCR_EL1_PA_SHIFT) |
>>  		    BIT(SYS_PMSCR_EL1_CX_SHIFT) |
>>  		    BIT(SYS_PMSCR_EL1_PCT_SHIFT))))
> 
> Acked-by: Will Deacon <[email-masked]>
> 
> Worth noting that this allows profiling of *physical* addresses used by
> memory access instructions and so probably has some security implications
> beyond the usual "but perf is buggy" line of reasoning.

Good to know. Thank you!
The data on physical addresses used by memory access instructions can already be
provided under CAP_SYS_ADMIN privileges [1] thus, I suppose, any implications you
have mentioned are already in place. I believe providing the data under CAP_PERFMON
alone without the rest of CAP_SYS_ADMIN credentials decreases chances to misuse the
data for harm and makes the monitoring more secure.

~Alexey

[1] https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html

> Will

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