KVM: nVMX: Add a sanity check that nested PML Full stems from EPT Violations

Add a WARN_ON_ONCE() sanity check to verify that a nested PML Full VM-Exit
is only synthesized when the original VM-Exit from L2 was an EPT Violation.
While KVM can fallthrough to kvm_mmu_do_page_fault() if an EPT Misconfig
occurs on a stale MMIO SPTE, KVM should not treat the access as a write
(there isn't enough information to know *what* the access was), i.e. KVM
should never try to insert a PML entry in that case.

Link: https://lore.kernel.org/r/20240209221700.393189-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/vmx/nested.c

@@ -416,6 +416,16 @@ static void nested_ept_inject_page_fault(struct kvm_vcpu *vcpu,
 		vm_exit_reason = EXIT_REASON_PML_FULL;
 		vmx->nested.pml_full = false;
 
+		/*
+		 * It should be impossible to trigger a nested PML Full VM-Exit
+		 * for anything other than an EPT Violation from L2.  KVM *can*
+		 * trigger nEPT page fault injection in response to an EPT
+		 * Misconfig, e.g. if the MMIO SPTE was stale and L1's EPT
+		 * tables also changed, but KVM should not treat EPT Misconfig
+		 * VM-Exits as writes.
+		 */
+		WARN_ON_ONCE(vmx->exit_reason.basic != EXIT_REASON_EPT_VIOLATION);
+
 		/*
 		 * PML Full and EPT Violation VM-Exits both use bit 12 to report
 		 * "NMI unblocking due to IRET", i.e. the bit can be propagated