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

Sai Prakash Ranjan
2021-01-19T08:41:29Z

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?

Thanks,
Sai

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