Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3

[Peter Zijlstra]

On Mon, Aug 21, 2017 at 06:56:01AM -0700, Andy Lutomirski wrote:
> 
> 
> > On Aug 21, 2017, at 3:33 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> >> 
> >> Using a kernel thread solves the problem for real.  Anything that
> >> blindly accesses user memory in kernel thread context is terminally
> >> broken no matter what.
> > 
> > So perf-callchain doesn't do it 'blindly', it wants either:
> > 
> > - user_mode(regs) true, or
> > - task_pt_regs() set.
> > 
> > However I'm thinking that if the kernel thread has ->mm == &efi_mm, the
> > EFI code running could very well have user_mode(regs) being true.
> > 
> > intel_pmu_pebs_fixup() OTOH 'blindly' assumes that the LBR addresses are
> > accessible. It bails on error though. So while its careful, it does
> > attempt to access the 'user' mapping directly. Which should also trigger
> > with the EFI code.
> > 
> > And I'm not seeing anything particularly broken with either. The PEBS
> > fixup relies on the CPU having just executed the code, and if it could
> > fetch and execute the code, why shouldn't it be able to fetch and read?
> 
> There are two ways this could be a problem.  One is that u privileged
> user apps shouldn't be able to read from EFI memory.

Ah, but only root can create per-cpu events or attach events to kernel
threads (with sensible paranoia levels).

> The other is that, if EFI were to have IO memory mapped at a "user"
> address, perf could end up reading it.

Ah, but in neither mode does perf assume much, the LBR follows branches
the CPU took and thus we _know_ there was code there, not MMIO. And the
stack unwind simply follows the stack up, although I suppose it could be
'tricked' into probing MMIO. We can certainly add an "->mm !=
->active_mm" escape clause to the unwind code.

Although I don't see how we're currently avoiding the same problem with
existing userspace unwinds, userspace can equally have MMIO mapped.

But neither will use pre-existing user addresses in the efi_mm I think.