KVM: x86/mmu: Move private vs. shared check above slot validity checks

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@linux.intel.com>
Cc: Chao Peng <chao.p.peng@linux.intel.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: Michael Roth <michael.roth@amd.com>
Cc: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Message-ID: <20240228024147.41573-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 9560fca3a6cf621f9b43831cedfd7f4075d45834..c5546cd7e1b93297aaed491ba0a19566cb9280fe 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4316,11 +4316,6 @@ 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);
 
@@ -4358,9 +4353,24 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 {
        int ret;
 
+       /*
+        * Note that the mmu_invalidate_seq also serves to detect a concurrent
+        * change in attributes.  is_page_fault_stale() will detect an
+        * invalidation relate to fault->fn and resume the guest without
+        * installing a mapping in the page tables.
+        */
        fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
        smp_rmb();
 
+       /*
+        * Now that we have a snapshot of mmu_invalidate_seq we can check for a
+        * private vs. shared mismatch.
+        */
+       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.