lkml.org 
[lkml]   [2014]   [May]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [RFC] x86_64: A real proposal for iret-less return to kernel
On Wed, May 21, 2014 at 3:36 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> 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.

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


\
 
 \ /
  Last update: 2014-05-22 01:21    [W:0.121 / U:0.496 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site