Re: [RFC] x86_64: A real proposal for iret-less return to kernel

["H. Peter Anvin" <hpa@zytor.com>]

On 05/21/2014 11:11 AM, Andy Lutomirski wrote:
> On Tue, May 20, 2014 at 5:53 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> Here's a real proposal for iret-less return.  If this is correct, then
>> NMIs will never nest, which will probably delete a lot more scariness
>> than is added by the code I'm describing.
> 
> OK, here's a case where I'm wrong.  An NMI interrupts userspace on a
> 16-bit stack.  The return from NMI goes through the espfix code.
> Something interrupts while on the espfix stack.  Boom!  Neither return
> style is particularly good.
> 
> More generally, if we got interrupted while on the espfix stack, we
> need to return back there using IRET.  Fortunately, re-enabling NMIs
> there in harmless, since we've already switched off the NMI stack.
> 
> This makes me think that maybe the logic should be turned around: have
> some RIP ranges on which the kernel stack might be invalid (which
> includes the espfix code and some of the syscall code) and use IRET
> only on return from NMI, return to nonstandard CS, and return to these
> special ranges.  The NMI code just needs to never so any of this stuff
> unless it switches off the NMI stack first.
> 
> For this to work reliably, we'll probably have to change CS before
> calling into EFI code.  That should be straightforward.
> 

I think you are onto something here.

In particular, the key observation here is that inside the kernel, we
can never *both* have an invalid stack *and* be inside an NMI, #MC or
#DB handler, even if nested.

Now, does this prevent us from using RET in the common case?  I'm not
sure it is a huge loss since kernel-to-kernel is relatively rare.

	-hpa