Re: [PATCH 41/70] x86/sev-es: Add Runtime #VC Exception Handler

Andy Lutomirski
2020-03-19T18:43:36Z

On Thu, Mar 19, 2020 at 9:24 AM Joerg Roedel <joro@8bytes.org> wrote:
>
> On Thu, Mar 19, 2020 at 08:44:03AM -0700, Andy Lutomirski wrote:
> > On Thu, Mar 19, 2020 at 2:14 AM Joerg Roedel <joro@8bytes.org> wrote:
> > >
> > > From: Tom Lendacky <thomas.lendacky@amd.com>
> > >
> > > Add the handler for #VC exceptions invoked at runtime.
> >
> > If I read this correctly, this does not use IST. If that's true, I
> > don't see how this can possibly work. There at least two nasty cases
> > that come to mind:
> >
> > 1. SYSCALL followed by NMI. The NMI IRET hack gets to #VC and we
> > explode. This is fixable by getting rid of the NMI EFLAGS.TF hack.
>
> Not an issue in this patch-set, the confusion comes from the fact that I
> left some parts of the single-step-over-iret code in the patch. But it
> is not used. The NMI handling in this patch-set sends the NMI-complete
> message before the IRET, when the kernel is still in a safe environment
> (kernel stack, kernel cr3).

Got it!

>
> > 2. tools/testing/selftests/x86/mov_ss_trap_64. User code does MOV
> > (addr), SS; SYSCALL, where addr has a data breakpoint. We get #DB
> > promoted to #VC with no stack.
>
> Also not an issue, as debugging is not supported at the moment in SEV-ES
> guests (hardware has no way yet to save/restore the debug registers
> across #VMEXITs). But this will change with future hardware. If you look
> at the implementation for dr7 read/write events, you see that the dr7
> value is cached and returned, but does not make it to the hardware dr7.

Eek. This would probably benefit from some ptrace / perf logic to
prevent the kernel or userspace from thinking that debugging works.

I guess this means that #DB only happens due to TF or INT01. I
suppose this is probably okay.

>
> I though about using IST for the #VC handler, but the implications for
> nesting #VC handlers made me decide against it. But for future hardware
> that supports debugging inside SEV-ES guests it will be an issue. I'll
> think about how to fix the problem, it probably has to be IST :(

Or future generations could have enough hardware support for debugging
that #DB doesn't need to be intercepted or can be re-injected
correctly with the #DB vector.

>
> Regards,
>
> Joerg