[PATCH 09/16] KVM: x86/mmu: Move private vs. shared check above slot validity checks

[Sean Christopherson]

Prioritize private vs. shared gfn attribute checks above slot validity
checks to ensure a consistent userspace ABI.  E.g. as is, KVM will exit to
userspace if there is no memslot, but emulate accesses to the APIC access
page even if the attributes mismatch.

Fixes: 8dd2eee9d526 ("KVM: x86/mmu: Handle page fault for private memory")
Cc: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
Cc: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
Cc: Fuad Tabba <tabba@xxxxxxxxxx>
Cc: Michael Roth <michael.roth@xxxxxxx>
Cc: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 9206cfa58feb..58c5ae8be66c 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4365,11 +4365,6 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 			return RET_PF_EMULATE;
 	}
 
-	if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
-		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
-		return -EFAULT;
-	}
-
 	if (fault->is_private)
 		return kvm_faultin_pfn_private(vcpu, fault);
 
@@ -4410,6 +4405,16 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 	fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
 	smp_rmb();
 
+	/*
+	 * Check for a private vs. shared mismatch *after* taking a snapshot of
+	 * mmu_invalidate_seq, as changes to gfn attributes are guarded by the
+	 * invalidation notifier.
+	 */
+	if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
+		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
+		return -EFAULT;
+	}
+
 	/*
 	 * Check for a relevant mmu_notifier invalidation event before getting
 	 * the pfn from the primary MMU, and before acquiring mmu_lock.
-- 
2.44.0.278.ge034bb2e1d-goog