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

Peter Zijlstra
2020-06-23T15:52:37Z

On Tue, Jun 23, 2020 at 04:39:26PM +0100, Andrew Cooper wrote:
> On 23/06/2020 16:23, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2020 at 04:59:14PM +0200, Joerg Roedel wrote:

> >> Yes, this is a start, it doesn't cover the case where the NMI stack is
> >> in-between, so I think you need to walk down regs->sp too.
> > That shouldn't be possible with the current code, I think.
>
> NMI; #MC; Anything which IRET but isn't fatal - #DB, or #BP from
> patching, #GP from *_safe(), etc; NMI
>
> Sure its a corner case, but did you hear that IST is evil?

Isn't current #MC unconditionally fatal from kernel? But yes, I was
sorta aware people want that changed.

And yes, NMI can recurse, mostly on #BP and #PF. Like I wrote, its
broken vs #MC.

But Joerg was talking about IST recursion with NMI in the middle,
something like: #DB, NMI, #DB, and not already being fatal. This one in
particular is ruled out by #DB itself clearing DR7 (but NMI would also
do that).

> P.S. did you also hear that with Rowhammer, userspace has a nonzero
> quantity of control over generating #MC, depending on how ECC is
> configured on the platform.

Yes, excellent stuff.