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

Andrew Cooper
2020-06-23T15:39:34Z

On 23/06/2020 16:23, 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.

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?

~Andrew

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.