Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 21 May 2014 11:11:41 -0700 | Subject | Re: [RFC] x86_64: A real proposal for iret-less return to kernel |
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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.
--Andy
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