linux-kernel - Re: [PATCH v3 18/20] perf: arm_spe: Fail device probe when arm64_kernel_unmapped_at

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Date:   Wed, 6 Dec 2017 13:34:51 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, ard.biesheuvel@...aro.org,
        sboyd@...eaurora.org, dave.hansen@...ux.intel.com,
        keescook@...omium.org, msalter@...hat.com, labbott@...hat.com,
        tglx@...utronix.de
Subject: Re: [PATCH v3 18/20] perf: arm_spe: Fail device probe when
 arm64_kernel_unmapped_at_el0()

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@....com>

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