Revert "KVM: MMU: fix validation of mmio page fault"

This reverts commit 41e3025e, which
was commit 6f691251 upstream.

The fix is only needed after commit f8f55942 ("KVM: MMU: fast
invalidate all mmio sptes"), included in Linux 3.11.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/x86/kvm/mmu.c

@@ -326,6 +326,12 @@ static u64 __get_spte_lockless(u64 *sptep)
 {
 	return ACCESS_ONCE(*sptep);
 }
+
+static bool __check_direct_spte_mmio_pf(u64 spte)
+{
+	/* It is valid if the spte is zapped. */
+	return spte == 0ull;
+}
 #else
 union split_spte {
 	struct {
@@ -430,6 +436,23 @@ static u64 __get_spte_lockless(u64 *sptep)
 
 	return spte.spte;
 }
+
+static bool __check_direct_spte_mmio_pf(u64 spte)
+{
+	union split_spte sspte = (union split_spte)spte;
+	u32 high_mmio_mask = shadow_mmio_mask >> 32;
+
+	/* It is valid if the spte is zapped. */
+	if (spte == 0ull)
+		return true;
+
+	/* It is valid if the spte is being zapped. */
+	if (sspte.spte_low == 0ull &&
+	    (sspte.spte_high & high_mmio_mask) == high_mmio_mask)
+		return true;
+
+	return false;
+}
 #endif
 
 static bool spte_has_volatile_bits(u64 spte)
@@ -2872,6 +2895,21 @@ static bool quickly_check_mmio_pf(struct kvm_vcpu *vcpu, u64 addr, bool direct)
 	return vcpu_match_mmio_gva(vcpu, addr);
 }
 
+
+/*
+ * On direct hosts, the last spte is only allows two states
+ * for mmio page fault:
+ *   - It is the mmio spte
+ *   - It is zapped or it is being zapped.
+ *
+ * This function completely checks the spte when the last spte
+ * is not the mmio spte.
+ */
+static bool check_direct_spte_mmio_pf(u64 spte)
+{
+	return __check_direct_spte_mmio_pf(spte);
+}
+
 static u64 walk_shadow_page_get_mmio_spte(struct kvm_vcpu *vcpu, u64 addr)
 {
 	struct kvm_shadow_walk_iterator iterator;
@@ -2912,6 +2950,13 @@ int handle_mmio_page_fault_common(struct kvm_vcpu *vcpu, u64 addr, bool direct)
 		return 1;
 	}
 
+	/*
+	 * It's ok if the gva is remapped by other cpus on shadow guest,
+	 * it's a BUG if the gfn is not a mmio page.
+	 */
+	if (direct && !check_direct_spte_mmio_pf(spte))
+		return -1;
+
 	/*
 	 * If the page table is zapped by other cpus, let CPU fault again on
 	 * the address.