Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)

Andrew Cooper
2020-04-28T16:34:44Z

On 28/04/2020 08:55, Joerg Roedel wrote:
> On Mon, Apr 27, 2020 at 10:37:41AM -0700, Andy Lutomirski wrote:
>> I have a somewhat serious question: should we use IST for #VC at all?
>> As I understand it, Rome and Naples make it mandatory for hypervisors
>> to intercept #DB, which means that, due to the MOV SS mess, it's sort
>> of mandatory to use IST for #VC. But Milan fixes the #DB issue, so,
>> if we're running under a sufficiently sensible hypervisor, we don't
>> need IST for #VC.
> The reason for #VC being IST is not only #DB, but also SEV-SNP. SNP adds
> page ownership tracking between guest and host, so that the hypervisor
> can't remap guest pages without the guest noticing.
>
> If there is a violation of ownership, which can happen at any memory
> access, there will be a #VC exception to notify the guest. And as this
> can happen anywhere, for example on a carefully crafted stack page set
> by userspace before doing SYSCALL, the only robust choice for #VC is to
> use IST.

The kernel won't ever touch the guest stack before restoring %rsp in the
syscall path, but the (minimum 2) memory accesses required to save the
user %rsp and load the kernel stack may be subject to #VC exceptions, as
are instruction fetches at the head of the SYSCALL path.

So yes - #VC needs IST.

Sorry for the noise.  (That said, it is unfortunate that the hypervisor
messing with the memory backing the guest #VC handler results in an
infinite loop, rather than an ability to cleanly terminate.)

~Andrew