linux-kernel - Re: [PATCHv2 0/4] perf/core: Add support to exclude kernel mode PMU tracing

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Date:   Tue, 9 Mar 2021 06:44:23 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     acme@...nel.org, al.grant@....com,
        alexander.shishkin@...ux.intel.com, coresight@...ts.linaro.org,
        denik@...omium.org, dianders@...omium.org, jolsa@...hat.com,
        leo.yan@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, mathieu.poirier@...aro.org,
        mike.leach@...aro.org, mingo@...hat.com, mnissler@...omium.org,
        namhyung@...nel.org, peterz@...radead.org, suzuki.poulose@....com,
        swboyd@...omium.org
Subject: Re: [PATCHv2 0/4] perf/core: Add support to exclude kernel mode PMU
 tracing

> The disk encryption is just one example and there might be others which
> we might not be aware of yet and we are not suspecting there is something
> wrong with the crypto code that needs to be fixed.

Then you don't have any leaks relating to branch tracing.

> restrict an external(in the sense that its not related to crypto or any
> other security related component) entity such as hardware assisted tracing
> like ARM coresight and so on. I don't see why or how the crypto code needs
> to be fixed for something that is not related to it although it is affected.

It's just a general property that if some code that is handling secrets
is data dependent it already leaks.


> The analogy would be like of the victims and a perpetrator. Lets take coresight
> as an example for perpetrator and crypto as the victim here. Now we can try

There's no victim with branch tracing, unless it is already leaky.

> If we just know one victim (lets say crypto code here), what happens to the
> others which we haven't identified yet? Do we just wait for someone to write
> an exploit based on this and then scramble to fix it?

For a useful security mitigation you need a threat model first I would say.

So you need to have at least some idea how an attack with branch
tracing would work.


> Initial change was to restrict this only to HW assisted instruction tracing [1]

I don't think it's needed for instruction tracing.

-Andi