Messages in this thread | | | Date | Wed, 21 May 2014 15:36:26 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [RFC] x86_64: A real proposal for iret-less return to kernel |
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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
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