Re: [PATCH v3 04/22] KVM: x86: Set vCPU exit reason to KVM_EXIT_UNKNOWN at the start of KVM_RUN

[Sean Christopherson <seanjc@google.com>]

On Tue, May 02, 2023, Anish Moorthy wrote:
> Thanks for nailing this down for me! One more question: should we be
> concerned about any guest memory accesses occurring in the preamble to
> that vcpu_run() call in kvm_arch_vcpu_ioctl_run()?
> 
> I only see two spots from which an EFAULT could make it to userspace,
> those being the sync_regs() and cui() calls. The former looks clean

Ya, sync_regs() is a non-issue, that doesn't touch guest memory unless userspace
is doing something truly bizarre.

> but I'm not sure about the latter. As written it's not an issue per se
> if the cui() call tries a vCPU memory access- the
> kvm_populate_efault_info() helper will just not populate the run
> struct and WARN_ON_ONCE(). But it would be good to know about.

If KVM triggers a WARN_ON_ONCE(), then that's an issue.  Though looking at the
code, the cui() aspect is a moot point.  As I stated in the previous discussion,
the WARN_ON_ONCE() in question needs to be off-by-default.

 : Hmm, one idea would be to have the initial -EFAULT detection fill kvm_run.memory_fault,
 : but set kvm_run.exit_reason to some magic number, e.g. zero it out.  Then KVM could
 : WARN if something tries to overwrite kvm_run.exit_reason.  The WARN would need to
 : be buried by a Kconfig or something since kvm_run can be modified by userspace,
 : but other than that I think it would work.