Re: [PATCH v2] x86: Return to kernel without IRET

[H. Peter Anvin]

On 05/03/2014 06:54 AM, Linus Torvalds wrote:
> On Fri, May 2, 2014 at 11:12 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> On 05/02/2014 09:32 PM, Linus Torvalds wrote:
>>>
>>> At least as a proof-of-concept, having a code sequence in user mode
>>> trampoline that does
>>>
>>>    popq %rsi
>>>    popq %r11
>>>    retq $128
>>>
>>> and building up a stack in user space at '%rsp-128' that has the
>>> values or rsi/r11/rip should allow us to use 'sysret'. Hmm?
>>
>> That would be a security hole if another userspace thread could muck
>> with the stack.
> 
> No, all of the above is in user space, and the pre-restore register
> values for rsi/r11/rip/rsp are all user space values (just not the
> right ones for the "real" return point). So no security issue.
> 
> Now, replacing "iret" with "sysret + user-space trampoline" doesn't
> work in general (it gets RF wrong, for example, so it's useless for
> single-stepping and breakpoint handling), but I was more thinking that
> it would be an interesting way to see what the performance impact of a
> faster iret would be.
> 

Right, brain failure on my part.  I somehow got it in my head you'd run
the above off the user stack while in CPL 0, which would be obviously crazy.

I think this would be rather interesting experiment.

	-hpa


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