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

[Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>]

On 2021-01-21 00:18, Mathieu Poirier wrote:
> On Tue, Jan 19, 2021 at 08:36:22AM +0000, 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.
>> 
>> 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 also think this is the right solution.
> 

Thanks for confirming, I will be working on this suggestion.

Thanks,
Sai

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