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

[Linus Torvalds]

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.

              Linus
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