Re: [PATCH v2] x86/viridian: Add Partition Reference Time enlightenment

[Ian Campbell <ian.campbell@citrix.com>]

On Mon, 2014-10-13 at 09:10 +0100, Jan Beulich wrote:
> >>> On 10.10.14 at 18:36, <msw@linux.com> wrote:
> > On Mon, Sep 29, 2014 at 11:28:44AM +0100, Paul Durrant wrote:
> >> +    /*
> >> +     * The guest will calculate reference time according to the following
> >> +     * formula:
> >> +     *
> >> +     * ReferenceTime = ((RDTSC() * TscScale) >> 64) + TscOffset
> >> +     *
> >> +     * Windows uses a 100ns tick, so we need a scale which is cpu
> >> +     * ticks per 100ns shifted left by 64.
> >> +     */
> >> +    p->TscScale = ((10000ul << 32) / d->arch.tsc_khz) << 32;
> >> +
> >> +    do {
> >> +        p->TscSequence++;
> >> +    } while ( p->TscSequence == 0xFFFFFFFF ||
> >> +              p->TscSequence == 0 ); /* Avoid both 'invalid' values */
> > 
> > Anthony Liguori and I were looking this over today and he pointed
> > something out: couldn't a second vCPU of the guest write 0 or
> > 0xffffffff in a tight loop to cause a hypervisor DoS?
> 
> Yes, this is at least a theoretical issue that should be fixed. I don't
> think it's a practical issue though: I'd expect the compiler to eliminate
> the two reads of the field and instead directly use the result of the
> increment.

Wouldn't that just mean the attacker needs to write fffffffe or ffffffff
instead?

Ian.