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

On Fri, Jan 15, 2021 at 11:16:24AM +0530, Sai Prakash Ranjan wrote:

Hello Mathieu, Suzuki

On 2020-10-15 21:32, Mathieu Poirier wrote:

quoted

On Thu, Oct 15, 2020 at 06:15:22PM +0530, Sai Prakash Ranjan wrote:

quoted

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.

Even if we were to apply this patch there are many ways to compromise a system
or get the kernel to reveal important information using the perf subsystem.  I
would perfer to tackle the problem at that level rather than concentrating on
coresight.

After this discussion, I would like to post a v2 based on Suzuki's
feedback earlier. @Suzuki, I have a common config for ETM3 and ETM4
but couldn't get much idea on how to implement it for Intel PTs, if
you have any suggestions there, please do share or we can have this
only for Coresight ETMs.

Thanks,
Sai

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