KVM: x86: Sanity check inputs to kvm_handle_memory_failure()

Add a sanity check in kvm_handle_memory_failure() to assert that a valid
x86_exception structure is provided if the memory "failure" wants to
propagate a fault into the guest.  If a memory failure happens during a
direct guest physical memory access, e.g. for nested VMX, KVM hardcodes
the failure to X86EMUL_IO_NEEDED and doesn't provide an exception pointer
(because the exception struct would just be filled with garbage).
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221220153427.514032-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -13132,6 +13132,9 @@ int kvm_handle_memory_failure(struct kvm_vcpu *vcpu, int r,
 			      struct x86_exception *e)
 {
 	if (r == X86EMUL_PROPAGATE_FAULT) {
+		if (KVM_BUG_ON(!e, vcpu->kvm))
+			return -EIO;
+
 		kvm_inject_emulated_page_fault(vcpu, e);
 		return 1;
 	}