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

[Sai Prakash Ranjan]

Hi Al,

On 2021-01-19 14:06, Al Grant wrote:
> Hi Sai,
>
> > From: saiprakash.ranjan=codeaurora.org@xxxxxxxxxxxxxxxxx
> > Hi Mathieu,
> >
> > On 2021-01-19 01:53, Mathieu Poirier wrote:
> > > 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.
> > >
> >
> > Sorry but I did not understand your point. We are talking about the 
> > capabilities
> > of coresight etm tracing which has the instruction level tracing and a 
> > lot more.
> > Perf subsystem is just the framework used for it.
> > In other words, its not the perf subsystem which does instruction 
> > level tracing,
> > its the coresight etm. Why the perf subsystem should be modified to 
> > lockdown
> > kernel mode? If we were to let perf handle all the trace filtering for 
> > different
> > exception levels, then why do we need the register settings in 
> > coresight etm
> > driver to filter out NS EL* tracing? And more importantly, how do you 
> > suppose
> > we handle sysfs mode of coresight tracing with perf subsystem?
>
> You both have good points. Mathieu is right that this is not a 
> CoreSight
> issue specifically, it is a matter of kernel security policy, and other 
> hardware
> tracing mechanisms ought to be within its scope. There should be a 
> general
> "anti kernel exfiltration" config that applies to all mechanisms within
> its scope, and we'd definitely expect that to include Intel PT as well 
> as ETM.

I agree with this part where there should be a generic config for all
hardware tracing families(atleast for Intel PT and ARM Coresight),
Suzuki suggested that as well. I am under the impression that Mathieu
didn't like adding such a config and wanted perf subsystem to handle
it since initial discussion was around whether root compromise meant
everything is lost already and such a kconfig would not help, but
Mattias already gave some good examples where that is not true.

> A kernel config that forced exclude_kernel on all perf events would 
> deal with
> ETM and PT in one place, but miss the sysfs interface to ETM.
>
> On the other hand, doing it in the ETM drivers would cover the perf and 
> sysfs
> interfaces to ETM, but would miss Intel PT.
>
> So I think what is needed is a general config option that is both 
> implemented
> in perf (excluding all kernel tracing events) and by any drivers that 
> provide
> an alternative interface to hardware tracing events.

I am good with this approach, once Mathieu confirms, I can add a kernel
wide kconfig as Suzuki suggested earlier and make ETM{3,4}x as the
initial users. Someone more familiar with Intel PTs can then make use
of this kconfig.

Thanks,
Sai

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