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

[Andy Lutomirski]

> On Aug 21, 2017, at 3:33 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
>> On Thu, Aug 17, 2017 at 08:52:38AM -0700, Andy Lutomirski wrote:
>> On Thu, Aug 17, 2017 at 3:35 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> 
>>> I'm still concerned that we're treating perf specially here -- are we
>>> absolutely sure that nobody else is going to attempt user accesses off the
>>> back of an interrupt?
>> 
>> Reasonably sure?  If nothing else, an interrupt taken while mmap_sem()
>> is held for write that tries to access user memory is asking for
>> serious trouble.  There are still a few callers of pagefault_disable()
>> and copy...inatomic(), though.
> 
> I'm not immediately seeing how holding mmap_sem for writing is a
> problem.
> 
>>> If not, then I'd much prefer a solution that catches
>>> anybody doing that with the EFI page table installed, rather than trying
>>> to play whack-a-mole like this.
>> 
>> 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.  The other is that, if EFI were to have IO memory mapped at a "user" address, perf could end up reading it.

> (eXecute implies Read assumed). And like said, it if triggers a fault,
> it bails, no worries.
> 
> It really doesn't care if the task is a kernel thread or not. Same for
> the unwinder, if we get an interrupt register set that points into
> 'userspace' we try and unwind it.