KVM: x86/mmu: WARN on an invalid root_hpa

WARN on the existing invalid root_hpa checks in __direct_map() and
FNAME(fetch).  The "legitimate" path that invalidated root_hpa in the
middle of a page fault is long since gone, i.e. it should no longer be
impossible to invalidate in the middle of a page fault[*].

The root_hpa checks were added by two related commits

  989c6b34 ("KVM: MMU: handle invalid root_hpa at __direct_map")
  37f6a4e2 ("KVM: x86: handle invalid root_hpa everywhere")

to fix a bug where nested_vmx_vmexit() could be called *in the middle*
of a page fault.  At the time, vmx_interrupt_allowed(), which was and
still is used by kvm_can_do_async_pf() via ->interrupt_allowed(),
directly invoked nested_vmx_vmexit() to switch from L2 to L1 to emulate
a VM-Exit on a pending interrupt.  Emulating the nested VM-Exit resulted
in root_hpa being invalidated by kvm_mmu_reset_context() without
explicitly terminating the page fault.

Now that root_hpa is checked for validity by kvm_mmu_page_fault(), WARN
on an invalid root_hpa to detect any flows that reset the MMU while
handling a page fault.  The broken vmx_interrupt_allowed() behavior has
long since been fixed and resetting the MMU during a page fault should
not be considered legal behavior.

[*] It's actually technically possible in FNAME(page_fault)() because it
    calls inject_page_fault() when the guest translation is invalid, but
    in that case the page fault handling is immediately terminated.
Signed-off-by: NSean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -3391,7 +3391,7 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t gpa, int write,
 	gfn_t gfn = gpa >> PAGE_SHIFT;
 	gfn_t base_gfn = gfn;
 
-	if (!VALID_PAGE(vcpu->arch.mmu->root_hpa))
+	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root_hpa)))
 		return RET_PF_RETRY;
 
 	if (likely(max_level > PT_PAGE_TABLE_LEVEL))

arch/x86/kvm/mmu/paging_tmpl.h

@@ -637,7 +637,7 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, gpa_t addr,
 	if (FNAME(gpte_changed)(vcpu, gw, top_level))
 		goto out_gpte_changed;
 
-	if (!VALID_PAGE(vcpu->arch.mmu->root_hpa))
+	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root_hpa)))
 		goto out_gpte_changed;
 
 	for (shadow_walk_init(&it, vcpu, addr);