Re: [syzbot] WARNING in nested_vmx_vmexit

Sean Christopherson
2021-12-06T17:21:56Z

On Mon, Dec 06, 2021, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> > I objected to the patch[*], but looking back at the dates, it appears that I did
> > so after the patch was queued and my comments were never addressed.
> > I'll see if I can reproduce this with a selftest. The fix is likely just:
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index dc4909b67c5c..927a7c43b73b 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -6665,10 +6665,6 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
> > * consistency check VM-Exit due to invalid guest state and bail.
> > */
> > if (unlikely(vmx->emulation_required)) {
> > -
> > - /* We don't emulate invalid state of a nested guest */
> > - vmx->fail = is_guest_mode(vcpu);
> > -
> > vmx->exit_reason.full = EXIT_REASON_INVALID_STATE;
> > vmx->exit_reason.failed_vmentry = 1;
> > kvm_register_mark_available(vcpu, VCPU_EXREG_EXIT_INFO_1);
> >
> > [*] https://lore.kernel.org/all/YWDWPbgJik5spT1D@google.com/
>
> Let's also summon Max to the discussion to get his thoughts.

Thinking more on this, we should do three things:

1. Revert this part back to "vmx->fail = 0".

2. Override SS.RPL and CS.RPL on RSM for VMX. Not sure this is strictly
necessary, I'm struggling to remember how SS.RPL and SS.DPL can get out of
sync.

IF RFLAGS.VM = 0 AND (in VMX root operation OR the “unrestricted guest”
VM-execution control is 0)
THEN
CS.RPL := SS.DPL;
SS.RPL := SS.DPL;
FI;

3. Modify RSM to go into TRIPLE_FAULT if vmx->emulation_required is true after
loading state for RSM. On AMD, whose SMRAM KVM emulates, all segment state
is read-only, i.e. if it's modified to be invalid then KVM essentially do
whatever it wants.

4. Reject KVM_RUN if is_guest_mode() and vmx->emulation_required are true. By
handling the RSM case explicitly, this means userspace has attempted to run
L2 with garbage, which KVM most definitely doesn't want to support.

5. Add KVM_BUG_ON(is_guest_mode(vcpu), ...) in the emulation_required path in
vmx_vcpu_run(), as reaching that point means KVM botched VM-Enter, RSM or
the #4 above.