On 05/21/2014 11:11 AM, Andy Lutomirski wrote:
> On Tue, May 20, 2014 at 5:53 PM, Andy Lutomirski <l...@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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
