linux-kernel - Re: [PATCH v2 18/18] perf: arm_spe: Disallow userspace profiling when arm_kernel_unmapped_at

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Message-ID: <20171201121505.5k2rif5hxsh4nu7f@lakrids.cambridge.arm.com>
Date:   Fri, 1 Dec 2017 12:15:06 +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 v2 18/18] perf: arm_spe: Disallow userspace profiling
 when arm_kernel_unmapped_at_el0()

On Thu, Nov 30, 2017 at 04:39:46PM +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.
> 
> This patch prohibits SPE profiling of userspace when
> arm_kernel_unmapped_at_el0().
> 
> Signed-off-by: Will Deacon <will.deacon@....com>
> ---
>  drivers/perf/arm_spe_pmu.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> index 8ce262fc2561..c028db8973a4 100644
> --- a/drivers/perf/arm_spe_pmu.c
> +++ b/drivers/perf/arm_spe_pmu.c
> @@ -675,6 +675,13 @@ static int arm_spe_pmu_event_init(struct perf_event *event)
>  		return -EOPNOTSUPP;
>  
>  	/*
> +	 * 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() && !attr->exclude_user)
> +		dev_warn_once(&spe_pmu->pdev->dev, "unable to write to profiling buffer from EL0. Try passing \"kaiser=off\" on the kernel command line");

The commit messages sats this prohibits profiling, but we simply log a
message.

I take it you meant to return an error code, too?

Thanks,
Mark.