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

["Natarajan, Janakarajan" <jnataraj@xxxxxxx>]

On 5/30/2018 2:24 AM, Jan Beulich wrote:
> > > > 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.
So check to see if there is a memory operand. Something along the lines of:

bool has_memory_operand(const struct x86_emulate_state *state,
                                                   const struct 
x86_emulate_ctxt *ctxt)
{
        if ( (state->desc & DstMask) == DstMem )
                return true;

        return false;
}

Thanks,
Janak



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