Messages in this thread | | | Date | Sat, 25 Apr 2020 14:49:09 +0200 | From | Joerg Roedel <> | Subject | Re: [PATCH] Allow RDTSC and RDTSCP from userspace |
| |
Hi Dave,
On Fri, Apr 24, 2020 at 03:53:09PM -0700, Dave Hansen wrote: > Ahh, so any instruction that can have an instruction intercept set > potentially needs to be able to tolerate a #VC? Those instruction > intercepts are under the control of the (untrusted relative to the > guest) hypervisor, right? > > >From the main sev-es series: > > +#ifdef CONFIG_AMD_MEM_ENCRYPT > +idtentry vmm_communication do_vmm_communication has_error_code=1 > +#endif
The next version of the patch-set (which I will hopefully have ready next week) will have this changed. The #VC exception handler uses an IST stack and is set to paranoid=1 and shift_ist. The IST stacks for the #VC handler are only allocated when SEV-ES is active.
> That's a fun point because it means that the (untrusted) hypervisor can > cause endless faults. I *guess* we have mitigation for this with our > stack guard pages, but it's still a bit nasty that the hypervisor can > arbitrarily land a guest in the double-fault handler. > > It just all seems a bit weak for the hypervisor to be considered > untrusted. But, it's _certainly_ a steep in the right direction from SEV.
Yeah, a malicious hypervisor can do bad things to an SEV-ES VM, but it can't easily steal its secrets from memory or registers. The #VC handler does its best to just crash the VM if unexpected hypervisor behavior is detected.
Regards,
Joerg
| |