Re: Tightening up rdpmc permissions?

[Peter Zijlstra]

On Mon, Sep 29, 2014 at 09:39:16AM -0700, Andy Lutomirski wrote:
> I was surprised to notice that, by default, every task has permission
> to use rdpmc. 

Right, we figured the paranoid would poke at
/sys/bus/event_source/devices/cpu/rdpmc.

> seccomp cannot work around this. 

I know nothing much about seccomp.

> This leaks information, although the information leaked is of dubious
> and variable value to an attacker.  It also renders the PR_TSC_SEGV
> mechanism mostly useless.

I'm not seeing how, there's no saying what will run on those counters,
let alone if its correlated to the TSC. But yes I appreciate the
argument.

> Would it make sense to restrict rdpmc to tasks that are in mms that
> have a perf_event mapping? 

We could,

> After all, unless I misunderstand
> something, user code can't reliably use rdpmc unless they've mapped a
> perf_event object to check the rdpmc bit and figure out what ecx value
> to use.

correct,

> I think that this could be implemented with very little overhead,
> especially if combined with the existing CR4_TSD code and if that code
> were taught to avoid reading cr4.

but there's a definite cost to having to write cr4 on every context
switch. It would be better if we could put it under TIF_NOTSC or
something similar and only be effective when something like seccomp or
other thingy is present and enabled. We should definitely avoid a double
cr4 write in case of both rdpmc and tsc switch.
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