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

[Mathieu Poirier]

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:
> > 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.

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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