linux-kernel - Re: [PATCH 1/4] perf/core: Add support to exclude kernel mode instruction tracing

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Date:   Fri, 29 Jan 2021 20:30:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Leo Yan <leo.yan@...aro.org>, coresight@...ts.linaro.org,
        Stephen Boyd <swboyd@...omium.org>,
        Denis Nikitin <denik@...omium.org>,
        Mattias Nissler <mnissler@...omium.org>,
        Al Grant <al.grant@....com>, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/4] perf/core: Add support to exclude kernel mode
 instruction tracing

On Sat, Jan 30, 2021 at 12:35:10AM +0530, Sai Prakash Ranjan wrote:

> Here the idea is to protect such important information from all users
> including root users since root privileges does not have to mean full
> control over the kernel [1] and root compromise does not have to be
> the end of the world.

And yet, your thing lacks:

> +config EXCLUDE_KERNEL_HW_ITRACE
> +	bool "Exclude kernel mode hardware assisted instruction tracing"
> +	depends on PERF_EVENTS
	depends on SECURITY_LOCKDOWN

or whatever the appropriate symbol is.

> +	help
> +	  Exclude kernel mode instruction tracing by hardware tracing
> +	  family such as ARM Coresight ETM, Intel PT and so on.
> +
> +	  This option allows to disable kernel mode instruction tracing
> +	  offered by hardware assisted tracing for all users(including root)
> +	  especially for production systems where only userspace tracing might
> +	  be preferred for security reasons.

Also, colour me unconvinced, pretty much all kernel level PMU usage
can be employed to side-channel / infer crypto keys, why focus on
ITRACE over others?