KVM: x86/mmu: Remove unnecessary ‘NULL’ values from sptep

Don't initialize "spte" and "sptep" in fast_page_fault() as they are both
guaranteed (for all intents and purposes) to be written at the start of
every loop iteration.  Add a sanity check that "sptep" is non-NULL after
walking the shadow page tables, as encountering a NULL root would result
in "spte" not being written, i.e. would lead to uninitialized data or the
previous value being consumed.

Signed-off-by: Li zeming <zeming@nfschina.com>
Link: https://lore.kernel.org/r/20230905182006.2964-1-zeming@nfschina.com
[sean: rewrite changelog with --verbose]
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/mmu.c

@@ -3425,8 +3425,8 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 {
 	struct kvm_mmu_page *sp;
 	int ret = RET_PF_INVALID;
-	u64 spte = 0ull;
-	u64 *sptep = NULL;
+	u64 spte;
+	u64 *sptep;
 	uint retry_count = 0;
 
 	if (!page_fault_can_be_fast(fault))
@@ -3442,6 +3442,14 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		else
 			sptep = fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
 
+		/*
+		 * It's entirely possible for the mapping to have been zapped
+		 * by a different task, but the root page should always be
+		 * available as the vCPU holds a reference to its root(s).
+		 */
+		if (WARN_ON_ONCE(!sptep))
+			spte = REMOVED_SPTE;
+
 		if (!is_shadow_present_pte(spte))
 			break;