Re: [Xen-devel] [PATCH v2 05/10] x86/SVM: Add AVIC vmexit handlers

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.05.18 at 01:33, <andrew.cooper3@xxxxxxxxxx> wrote:
>> Would this be better suited ?
> 
> Almost.
> 
> The purpose of the validate function is to fix an inherent race
> condition which occurs with a vmexit.
> 
> After a vmexit, rereading the instruction for emulation is inherently
> racy, and a malicious VM could rewrite the instruction stream while the
> vmexit is occuring.  As a result, we provide a validate function to
> check that the instruction decoded matches one which plausibly broke for
> emulation here.
> 
> Therefore, you want a validate function which checks that the
> instruction has a memory operand, and that it falls within the 4k region
> which maps the APIC registers.

The validate hook is called right after decode, i.e. before operands have
been evaluated, so the latter part of what you suggest cannot be done.

Jan



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