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

[Andy Lutomirski]

On Wed, May 21, 2014 at 3:36 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 05/21/2014 11:11 AM, Andy Lutomirski wrote:
>> On Tue, May 20, 2014 at 5:53 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.

Except for espfix :)

>
> 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.

I don't think so.  The most common case should be plain old interrupts
and I suspect that #PF is a distant second.

In any event, plain old interrupts and #PF are non-IST interrupts and
they should be unconditionally safe for RET

--Andy
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