Re: [PATCH 1/4] perf/core: Add support to exclude kernel mode instruction tracing

[Sai Prakash Ranjan]

Hi Peter,

On 2021-02-02 11:41, Sai Prakash Ranjan wrote:
> Hi Peter,
>
> On 2021-02-01 19:11, Peter Zijlstra wrote:
> > On Mon, Feb 01, 2021 at 01:11:04PM +0530, Sai Prakash Ranjan wrote:
> >
> > > Ok I suppose you mean CONFIG_SECURITY_LOCKDOWN_LSM? But I don't see
> > > how this new config has to depend on that? This can work 
> > > independently
> > > whether complete lockdown is enforced or not since it applies to only
> > > hardware instruction tracing. Ideally this depends on several 
> > > hardware
> > > tracing configs such as ETMs and others but we don't need them 
> > > because
> > > we are already exposing PERF_PMU_CAP_ITRACE check in the events core.
> >
> > If you don't have lockdown, root pretty much owns the kernel, or am I
> > missing something?
>
> You are right in saying that without lockdown root would own kernel but
> this config(EXCLUDE_KERNEL) will independently make sure that kernel
> level pmu tracing is not allowed(we return -EACCES) even if LOCKDOWN
> config is disabled. So I'm saying that we don't need to depend on
> LOCKDOWN config, its good to have LOCKDOWN config enabled but perf
> subsystem doesn't have to care about that.
>
> > > be used for some speculative execution based attacks. Which other
> > > kernel level PMUs can be used to get a full branch trace that is not
> > > locked down? If there is one, then this should probably be applied to
> > > it as well.
> >
> > Just the regular counters. The information isn't as accurate, but 
> > given
> > enough goes you can infer plenty.
> >
> > Just like all the SMT size-channel attacks.
> >
> > Sure, PT and friends make it even easier, but I don't see a 
> > fundamental
> > distinction.
>
> Right, we should then exclude all kernel level pmu tracing, is it fine?
>
> if (IS_ENABLED(CONFIG_EXCLUDE_KERNEL_HW_ITRACE) && 
> !attr.exclude_kernel))
>     return -EACCES;

Sorry for being pushy, but does the above make sense?

Thanks,
Sai

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