KVM: x86/mmu: Set SPTE_AD_WRPROT_ONLY_MASK if and only if PML is enabled

Check that PML is actually enabled before setting the mask to force a
SPTE to be write-protected.  The bits used for the !AD_ENABLED case are
in the upper half of the SPTE.  With 64-bit paging and EPT, these bits
are ignored, but with 32-bit PAE paging they are reserved.  Setting them
for L2 SPTEs without checking PML breaks NPT on 32-bit KVM.

Fixes: 1f4e5fc8 ("KVM: x86: fix nested guest live migration with PML")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu_internal.h

@@ -81,15 +81,15 @@ static inline struct kvm_mmu_page *sptep_to_sp(u64 *sptep)
 static inline bool kvm_vcpu_ad_need_write_protect(struct kvm_vcpu *vcpu)
 {
 	/*
-	 * When using the EPT page-modification log, the GPAs in the log
-	 * would come from L2 rather than L1.  Therefore, we need to rely
-	 * on write protection to record dirty pages.  This also bypasses
-	 * PML, since writes now result in a vmexit.  Note, this helper will
-	 * tag SPTEs as needing write-protection even if PML is disabled or
-	 * unsupported, but that's ok because the tag is consumed if and only
-	 * if PML is enabled.  Omit the PML check to save a few uops.
+	 * When using the EPT page-modification log, the GPAs in the CPU dirty
+	 * log would come from L2 rather than L1.  Therefore, we need to rely
+	 * on write protection to record dirty pages, which bypasses PML, since
+	 * writes now result in a vmexit.  Note, the check on CPU dirty logging
+	 * being enabled is mandatory as the bits used to denote WP-only SPTEs
+	 * are reserved for NPT w/ PAE (32-bit KVM).
 	 */
-	return vcpu->arch.mmu == &vcpu->arch.guest_mmu;
+	return vcpu->arch.mmu == &vcpu->arch.guest_mmu &&
+	       kvm_x86_ops.cpu_dirty_log_size;
 }
 
 bool is_nx_huge_page_enabled(void);