Re: Virt Call depth tracking mitigation

Thomas Gleixner
2022-07-19T21:17:19Z

On Tue, Jul 19 2022 at 16:23, Andrew Cooper wrote:
> On 19/07/2022 15:13, Thomas Gleixner wrote:
>> Well, that makes the guest side "safe", but isn't a deep hypercall > 16
>> already underflowing in the hypervisor code before it returns to the
>> guest?
>
> Yeah, but that's the hypervisor's problem to deal with, in whatever
> manner it sees fit.
>
> And if the hypervisor is using IBeeRS then the first ret in guest
> context will underflow.

I have a look tomorrow.

>>> That said, after a return from the hypervisor, Linux has no idea what
>>> state the RSB is in, so the only safe course of action is to re-stuff.
>> Indeed.
>>
>> Another proof for my claim that virt creates more problems than it
>> solves.
>
> So how did you like debugging the gsbase crash on native hardware. :)

First of all I said it's creating more problems than it solves, which
means it solves some problems.

But more important, I'm not a wimp.

Thanks,

tglx