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

[Will Deacon]

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 <alexey.budankov at linux.intel.com>
> ---
>  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 <will at kernel.org>

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.

Will