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

Suzuki K Poulose
2021-01-19T10:49:47Z

On 1/19/21 9:51 AM, Sai Prakash Ranjan wrote:
> 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.

Instead of adding the support for individual drivers, you could handle this
in the generic perf layer. e.g, Fail perf_event create with an attribute
which allows kernel tracing ?

if (!attr.exclude_kernel)
return -EINVAL;

Or even exclude the kernel silently always.

This could also be limited to PMUs with PERF_PMU_CAP_ITRACE, if you
want to limit this to PMUs that instruction level tracing.

Suzuki