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

Joerg Roedel
2020-06-23T15:39:00Z

On Tue, Jun 23, 2020 at 05:23:26PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 23, 2020 at 04:59:14PM +0200, Joerg Roedel wrote:
> > On Tue, Jun 23, 2020 at 04:53:44PM +0200, Peter Zijlstra wrote:
> > > +noinstr void idtentry_validate_ist(struct pt_regs *regs)
> > > +{
> > > + if ((regs->sp & ~(EXCEPTION_STKSZ-1)) ==
> > > + (_RET_IP_ & ~(EXCEPTION_STKSZ-1)))
> > > + die("IST stack recursion", regs, 0);
> > > +}
> >
> > 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.

Not with the current code, but possibly with SNP #VC exceptions:

-> First #VC
-> NMI before VC handler switched off its IST stack
(now on NMI IST stack)
-> Second SNP #VC exception before the NMI handler did the
#VC stack check (because HV messed around with some pages
touched there).

In the second #VC you use the same IST stack as in the first #VC, but
the the NMI-stack in-between.

> Reliability of that depends on the unwinder, I wouldn't want the guess
> uwinder to OOPS me by accident.

It doesn't use the full unwinder, it just assumes that there is a
pt_regs struct at the top of every kernel stack and walks through them
until SP points to a user-space stack.

As long as the assumption that there is a pt_regs struct on top of every
stack holds, this should be safe. The assumption might be wrong when an
exception happens during SYSCALL/SYSENTER entry, when the return frame
is not written by hardware.

Joerg