linux-kernel - Re: [PATCH] coresight: etm4x: Add config to exclude kernel mode tracing

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Date:   Tue, 19 Jan 2021 10:42:47 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Mattias Nissler <mnissler@...omium.org>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>, coresight@...ts.linaro.org,
        Stephen Boyd <swboyd@...omium.org>,
        Denis Nikitin <denik@...omium.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, Al Grant <al.grant@....com>,
        leo.yan@...aro.org
Subject: Re: [PATCH] coresight: etm4x: Add config to exclude kernel mode
 tracing

On 2021-01-18 20:17, Mattias Nissler wrote:
> On Fri, Jan 15, 2021 at 6:46 AM Sai Prakash Ranjan
> <saiprakash.ranjan@...eaurora.org> wrote:
>> 
>> Hello Mathieu, Suzuki
>> 
>> On 2020-10-15 21:32, Mathieu Poirier wrote:
>> > On Thu, Oct 15, 2020 at 06:15:22PM +0530, Sai Prakash Ranjan wrote:
>> >> On production systems with ETMs enabled, it is preferred to
>> >> exclude kernel mode(NS EL1) tracing for security concerns and
>> >> support only userspace(NS EL0) tracing. So provide an option
>> >> via kconfig to exclude kernel mode tracing if it is required.
>> >> This config is disabled by default and would not affect the
>> >> current configuration which has both kernel and userspace
>> >> tracing enabled by default.
>> >>
>> >
>> > One requires root access (or be part of a special trace group) to be
>> > able to use
>> > the cs_etm PMU.  With this kind of elevated access restricting tracing
>> > at EL1
>> > provides little in terms of security.
>> >
>> 
>> Apart from the VM usecase discussed, I am told there are other
>> security concerns here regarding need to exclude kernel mode tracing
>> even for the privileged users/root. One such case being the ability
>> to analyze cryptographic code execution since ETMs can record all
>> branch instructions including timestamps in the kernel and there may
>> be other cases as well which I may not be aware of and hence have
>> added Denis and Mattias. Please let us know if you have any questions
>> further regarding this not being a security concern.
> 
> Well, the idea that root privileges != full control over the kernel
> isn't new and at the very least since lockdown became part of mainline
> [1] no longer an esoteric edge case. Regarding the use case Sai hints
> at (namely protection of secrets in the kernel), Matthew Garret
> actually has some more thoughts about confidentiality mode for
> lockdown for secret protection [2]. And thus, unless someone can make
> a compelling case that instruction-level tracing will not leak secrets
> held by the kernel, I think an option for the kernel to prevent itself
> from being traced (even by root) is valuable.
> 
> Finally, to sketch a practical use case scenario: Consider a system
> where disk contents are encrypted and the encryption key is set up by
> the user when mounting the file system. From that point on the
> encryption key resides in the kernel. It seems reasonable to expect
> that the disk encryption key be protected from exfiltration even if
> the system later suffers a root compromise (or even against insiders
> that have root access), at least as long as the attacker doesn't
> manage to compromise the kernel.
> 
> [1] https://lwn.net/Articles/796866/
> [2] https://mjg59.dreamwidth.org/55105.html
> 

Thanks for the detailed description, it is way better put than my crude
explanation.

Thanks,
Sai

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