Re: [RFC PATCH 3/3] KVM: x86: Use KVM_BUG/KVM_BUG_ON to handle bugs that are fatal to the VM

Sean Christopherson
2020-09-25T17:25:43Z

On Fri, Sep 25, 2020 at 11:50:38AM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@intel.com> writes:
>
> > On Thu, Sep 24, 2020 at 02:34:14PM +0200, Vitaly Kuznetsov wrote:
> >> Sean Christopherson <sean.j.christopherson@intel.com> writes:
> >> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> >> > index 6f9a0c6d5dc5..810d46ab0a47 100644
> >> > --- a/arch/x86/kvm/vmx/vmx.c
> >> > +++ b/arch/x86/kvm/vmx/vmx.c
> >> > @@ -4985,14 +4986,13 @@ static int handle_cr(struct kvm_vcpu *vcpu)
> >> > }
> >> > break;
> >> > case 2: /* clts */
> >> > - WARN_ONCE(1, "Guest should always own CR0.TS");
> >> > - vmx_set_cr0(vcpu, kvm_read_cr0_bits(vcpu, ~X86_CR0_TS));
> >> > - trace_kvm_cr_write(0, kvm_read_cr0(vcpu));
> >> > - return kvm_skip_emulated_instruction(vcpu);
> >> > + KVM_BUG(1, vcpu->kvm, "Guest always owns CR0.TS");
> >> > + return -EIO;
> >> > case 1: /*mov from cr*/
> >> > switch (cr) {
> >> > case 3:
> >> > WARN_ON_ONCE(enable_unrestricted_guest);
> >> > +
> >>
> >> Here, were you intended to replace WARN_ON_ONCE() with KVM_BUG_ON() or
> >> this is just a stray newline added?
> >
> > I think it's just a stray newline. At one point I had converted this to a
> > KVM_BUG_ON(), but then reversed direction because it's not fatal to the guest,
> > i.e. KVM should continue to function even though it's spuriously intercepting
> > CR3 loads.
> >
> > Which, rereading this patch, completely contradicts the KVM_BUG() for CLTS.
> >
> > That's probably something we should sort out in this RFC: is KVM_BUG() only
> > to be used if the bug is fatal/dangerous, or should it be used any time the
> > error is definitely a KVM (or hardware) bug.
>
> Personally, I'm feeling adventurous so my vote goes to the later :-)
> Whenever a KVM bug was discovered by a VM it's much safer to stop
> executing it as who knows what the implications might be?

Not necessarily, e.g. terminating the VM may corrupt the VM's file system,
which is less safe, for lack of a better word, from the VM's perspective.

> In this particular case I can think of a nested scenario when L1 didn't
> ask for CR3 intercept but L0 is still injecting it. It is not fatal by
> itself but probably there is bug in calculating intercepts in L0 so
> if we're getting something extra maybe we're also missing some? And this
> doesn't sound good at all.

Hmm, but by that argument this scenario would fall into the "dangerous" part
of "bug is fatal/dangerous". I guess my opinion is that we should set a
fairly high bar for using KVM_BUG() so that KVM can be aggressive in shutting
down.

> > In theory, it should be impossible to reach this again as "r = -EIO" will
> > bounce this out to userspace, the common checks to deny all ioctls() will
> > prevent reinvoking KVM_RUN.
>
> Do we actually want to prevent *all* ioctls? E.g. when 'vm bugged'
> condition is triggered userspace may want to extract some information to
> assist debugging but even things like KVM_GET_[S]REGS will just return
> -EIO. I'm not sure it is generally safe to enable *everything* (except
> for KVM_RUN which should definitely be forbidden) so maybe your approach
> is preferable.

The answer to this probably depends on the answer to the first question of
when it's appropriate to use KVM_BUG(). E.g. if we limit usage to fatal or
dangrous cases, then blocking all ioctls() is probably the right thing do do.