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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.10.14 at 18:36, <msw@xxxxxxxxx> 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.

Jan


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