KVM: X86: Fix SMRAM accessing even if VM is shutdown

Reported by syzkaller:

   WARNING: CPU: 6 PID: 2434 at arch/x86/kvm/vmx.c:6660 handle_ept_misconfig+0x54/0x1e0 [kvm_intel]
   CPU: 6 PID: 2434 Comm: repro_test Not tainted 4.15.0+ #4
   RIP: 0010:handle_ept_misconfig+0x54/0x1e0 [kvm_intel]
   Call Trace:
    vmx_handle_exit+0xbd/0xe20 [kvm_intel]
    kvm_arch_vcpu_ioctl_run+0xdaf/0x1d50 [kvm]
    kvm_vcpu_ioctl+0x3e9/0x720 [kvm]
    do_vfs_ioctl+0xa4/0x6a0
    SyS_ioctl+0x79/0x90
    entry_SYSCALL_64_fastpath+0x25/0x9c

The testcase creates a first thread to issue KVM_SMI ioctl, and then creates
a second thread to mmap and operate on the same vCPU.  This triggers a race
condition when running the testcase with multiple threads. Sometimes one thread
exits with a triple fault while another thread mmaps and operates on the same
vCPU.  Because CS=0x3000/IP=0x8000 is not mapped, accessing the SMI handler
results in an EPT misconfig. This patch fixes it by returning RET_PF_EMULATE
in kvm_handle_bad_page(), which will go on to cause an emulation failure and an
exit with KVM_EXIT_INTERNAL_ERROR.

Reported-by: syzbot+c1d9517cab094dae65e446c0c5b4de6c40f4dc58@syzkaller.appspotmail.com
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu.c

@@ -3029,7 +3029,7 @@ static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
 		return RET_PF_RETRY;
 	}
 
-	return -EFAULT;
+	return RET_PF_EMULATE;
 }
 
 static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,