KVM: x86/mmu: Check SPTE writable invariants when setting leaf SPTEs

Check SPTE writable invariants when setting SPTEs rather than in
spte_can_locklessly_be_made_writable(). By the time KVM checks
spte_can_locklessly_be_made_writable(), the SPTE has long been since
corrupted.

Note that these invariants only apply to shadow-present leaf SPTEs (i.e.
not to MMIO SPTEs, non-leaf SPTEs, etc.). Add a comment explaining the
restriction and only instrument the code paths that set shadow-present
leaf SPTEs.

To account for access tracking, also check the SPTE writable invariants
when marking an SPTE as an access track SPTE. This also lets us remove
a redundant WARN from mark_spte_for_access_track().
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220125230518.1697048-3-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -529,6 +529,7 @@ static u64 mmu_spte_update_no_track(u64 *sptep, u64 new_spte)
 	u64 old_spte = *sptep;
 
 	WARN_ON(!is_shadow_present_pte(new_spte));
+	check_spte_writable_invariants(new_spte);
 
 	if (!is_shadow_present_pte(old_spte)) {
 		mmu_spte_set(sptep, new_spte);

arch/x86/kvm/mmu/spte.c

@@ -250,14 +250,7 @@ u64 mark_spte_for_access_track(u64 spte)
 	if (is_access_track_spte(spte))
 		return spte;
 
-	/*
-	 * Making an Access Tracking PTE will result in removal of write access
-	 * from the PTE. So, verify that we will be able to restore the write
-	 * access in the fast page fault path later on.
-	 */
-	WARN_ONCE((spte & PT_WRITABLE_MASK) &&
-		  !spte_can_locklessly_be_made_writable(spte),
-		  "kvm: Writable SPTE is not locklessly dirty-trackable\n");
+	check_spte_writable_invariants(spte);
 
 	WARN_ONCE(spte & (SHADOW_ACC_TRACK_SAVED_BITS_MASK <<
 			  SHADOW_ACC_TRACK_SAVED_BITS_SHIFT),

arch/x86/kvm/mmu/spte.h

@@ -339,6 +339,7 @@ static __always_inline bool is_rsvd_spte(struct rsvd_bits_validate *rsvd_check,
 	       __is_rsvd_bits_set(rsvd_check, spte, level);
 }
 
+/* Note: spte must be a shadow-present leaf SPTE. */
 static inline void check_spte_writable_invariants(u64 spte)
 {
 	if (spte & shadow_mmu_writable_mask)
@@ -352,7 +353,6 @@ static inline void check_spte_writable_invariants(u64 spte)
 
 static inline bool spte_can_locklessly_be_made_writable(u64 spte)
 {
-	check_spte_writable_invariants(spte);
 	return spte & shadow_mmu_writable_mask;
 }
 

arch/x86/kvm/mmu/tdp_mmu.c

@@ -452,6 +452,9 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
 
 	trace_kvm_tdp_mmu_spte_changed(as_id, gfn, level, old_spte, new_spte);
 
+	if (is_leaf)
+		check_spte_writable_invariants(new_spte);
+
 	/*
 	 * The only times a SPTE should be changed from a non-present to
 	 * non-present state is when an MMIO entry is installed/modified/