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

Sai Prakash Ranjan
2021-01-19T10:13:44Z

Hi Al,

On 2021-01-19 14:06, Al Grant wrote:
> Hi Sai,
>
>> From: saiprakash.ranjan=codeaurora.org@mg.codeaurora.org
>> 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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