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

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Date:   Wed, 10 Feb 2021 13:08:05 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Peter Zijlstra <peterz@...radead.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,
        jannh@...gle.com
Subject: Re: [PATCH 1/4] perf/core: Add support to exclude kernel mode
 instruction tracing

Hi Peter,

On 2021-02-02 11:41, Sai Prakash Ranjan wrote:
> Hi Peter,
> 
> On 2021-02-01 19:11, Peter Zijlstra wrote:
>> On Mon, Feb 01, 2021 at 01:11:04PM +0530, Sai Prakash Ranjan wrote:
>> 
>>> Ok I suppose you mean CONFIG_SECURITY_LOCKDOWN_LSM? But I don't see
>>> how this new config has to depend on that? This can work 
>>> independently
>>> whether complete lockdown is enforced or not since it applies to only
>>> hardware instruction tracing. Ideally this depends on several 
>>> hardware
>>> tracing configs such as ETMs and others but we don't need them 
>>> because
>>> we are already exposing PERF_PMU_CAP_ITRACE check in the events core.
>> 
>> If you don't have lockdown, root pretty much owns the kernel, or am I
>> missing something?
>> 
> 
> You are right in saying that without lockdown root would own kernel but
> this config(EXCLUDE_KERNEL) will independently make sure that kernel
> level pmu tracing is not allowed(we return -EACCES) even if LOCKDOWN
> config is disabled. So I'm saying that we don't need to depend on
> LOCKDOWN config, its good to have LOCKDOWN config enabled but perf
> subsystem doesn't have to care about that.
> 
>>> be used for some speculative execution based attacks. Which other
>>> kernel level PMUs can be used to get a full branch trace that is not
>>> locked down? If there is one, then this should probably be applied to
>>> it as well.
>> 
>> Just the regular counters. The information isn't as accurate, but 
>> given
>> enough goes you can infer plenty.
>> 
>> Just like all the SMT size-channel attacks.
>> 
>> Sure, PT and friends make it even easier, but I don't see a 
>> fundamental
>> distinction.
> 
> Right, we should then exclude all kernel level pmu tracing, is it fine?
> 
> if (IS_ENABLED(CONFIG_EXCLUDE_KERNEL_HW_ITRACE) && 
> !attr.exclude_kernel))
>     return -EACCES;
> 

Sorry for being pushy, but does the above make sense?

Thanks,
Sai

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