Re: [PATCH v3 18/20] perf: arm_spe: Fail device probe when arm64_kernel_unmapped_at_el0()

Mark Rutland
2017-12-06T13:34:58Z

On Wed, Dec 06, 2017 at 12:35:37PM +0000, Will Deacon wrote:
> When running with the kernel unmapped whilst at EL0, the virtually-addressed
> SPE buffer is also unmapped, which can lead to buffer faults if userspace
> profiling is enabled and potentially also when writing back kernel samples
> unless an expensive drain operation is performed on exception return.
>
> For now, fail the SPE driver probe when arm64_kernel_unmapped_at_el0().
>
> Signed-off-by: Will Deacon <will.deacon@arm.com>

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
> drivers/perf/arm_spe_pmu.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> index 8ce262fc2561..51b40aecb776 100644
> --- a/drivers/perf/arm_spe_pmu.c
> +++ b/drivers/perf/arm_spe_pmu.c
> @@ -1164,6 +1164,15 @@ static int arm_spe_pmu_device_dt_probe(struct platform_device *pdev)
> struct arm_spe_pmu *spe_pmu;
> struct device *dev = &pdev->dev;
>
> + /*
> + * If kernelspace is unmapped when running at EL0, then the SPE
> + * buffer will fault and prematurely terminate the AUX session.
> + */
> + if (arm64_kernel_unmapped_at_el0()) {
> + dev_warn_once(dev, "profiling buffer inaccessible. Try passing \"kpti=off\" on the kernel command line\n");
> + return -EPERM;
> + }
> +
> spe_pmu = devm_kzalloc(dev, sizeof(*spe_pmu), GFP_KERNEL);
> if (!spe_pmu) {
> dev_err(dev, "failed to allocate spe_pmu\n");
> --
> 2.1.4
>