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

Peter Zijlstra
2020-06-23T15:23:56Z

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.

> The dumpstack code already has some logic for this.

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