KVM: MMU: Disable fast path if KVM_EXIT_MEMORY_FAULT is needed

For hardware-protected VMs like SEV-SNP guests, certain conditions like
attempting to perform a write to a page which is not in the state that
the guest expects it to be in can result in a nested/extended #PF which
can only be satisfied by the host performing an implicit page state
change to transition the page into the expected shared/private state.
This is generally handled by generating a KVM_EXIT_MEMORY_FAULT event
that gets forwarded to userspace to handle via
KVM_SET_MEMORY_ATTRIBUTES.

However, the fast_page_fault() code might misconstrue this situation as
being the result of a write-protected access, and treat it as a spurious
case when it sees that writes are already allowed for the sPTE. This
results in the KVM MMU trying to resume the guest rather than taking any
action to satisfy the real source of the #PF such as generating a
KVM_EXIT_MEMORY_FAULT, resulting in the guest spinning on nested #PFs.

Check for this condition and bail out of the fast path if it is
detected.
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: Isaku Yamahata <isaku.yamahata@intel.com>
Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -3305,7 +3305,7 @@ static int kvm_handle_noslot_fault(struct kvm_vcpu *vcpu,
 	return RET_PF_CONTINUE;
 }
 
-static bool page_fault_can_be_fast(struct kvm_page_fault *fault)
+static bool page_fault_can_be_fast(struct kvm *kvm, struct kvm_page_fault *fault)
 {
 	/*
 	 * Page faults with reserved bits set, i.e. faults on MMIO SPTEs, only
@@ -3316,6 +3316,26 @@ static bool page_fault_can_be_fast(struct kvm_page_fault *fault)
 	if (fault->rsvd)
 		return false;
 
+	/*
+	 * For hardware-protected VMs, certain conditions like attempting to
+	 * perform a write to a page which is not in the state that the guest
+	 * expects it to be in can result in a nested/extended #PF. In this
+	 * case, the below code might misconstrue this situation as being the
+	 * result of a write-protected access, and treat it as a spurious case
+	 * rather than taking any action to satisfy the real source of the #PF
+	 * such as generating a KVM_EXIT_MEMORY_FAULT. This can lead to the
+	 * guest spinning on a #PF indefinitely, so don't attempt the fast path
+	 * in this case.
+	 *
+	 * Note that the kvm_mem_is_private() check might race with an
+	 * attribute update, but this will either result in the guest spinning
+	 * on RET_PF_SPURIOUS until the update completes, or an actual spurious
+	 * case might go down the slow path. Either case will resolve itself.
+	 */
+	if (kvm->arch.has_private_mem &&
+	    fault->is_private != kvm_mem_is_private(kvm, fault->gfn))
+		return false;
+
 	/*
 	 * #PF can be fast if:
 	 *
@@ -3416,7 +3436,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 	u64 *sptep;
 	uint retry_count = 0;
 
-	if (!page_fault_can_be_fast(fault))
+	if (!page_fault_can_be_fast(vcpu->kvm, fault))
 		return ret;
 
 	walk_shadow_page_lockless_begin(vcpu);