KVM: Don't enable hardware after a restart/shutdown is initiated

Reject hardware enabling, i.e. VM creation, if a restart/shutdown has
been initiated to avoid re-enabling hardware between kvm_reboot() and
machine_{halt,power_off,restart}().  The restart case is especially
problematic (for x86) as enabling VMX (or clearing GIF in KVM_RUN on
SVM) blocks INIT, which results in the restart/reboot hanging as BIOS
is unable to wake and rendezvous with APs.

Note, this bug, and the original issue that motivated the addition of
kvm_reboot(), is effectively limited to a forced reboot, e.g. `reboot -f`.
In a "normal" reboot, userspace will gracefully teardown userspace before
triggering the kernel reboot (modulo bugs, errors, etc), i.e. any process
that might do ioctl(KVM_CREATE_VM) is long gone.

Fixes: 8e1c1815 ("KVM: VMX: Disable VMX when system shutdown")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Message-Id: <20230512233127.804012-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -5184,7 +5184,20 @@ static void hardware_disable_all(void)
 static int hardware_enable_all(void)
 {
 	atomic_t failed = ATOMIC_INIT(0);
-	int r = 0;
+	int r;
+
+	/*
+	 * Do not enable hardware virtualization if the system is going down.
+	 * If userspace initiated a forced reboot, e.g. reboot -f, then it's
+	 * possible for an in-flight KVM_CREATE_VM to trigger hardware enabling
+	 * after kvm_reboot() is called.  Note, this relies on system_state
+	 * being set _before_ kvm_reboot(), which is why KVM uses a syscore ops
+	 * hook instead of registering a dedicated reboot notifier (the latter
+	 * runs before system_state is updated).
+	 */
+	if (system_state == SYSTEM_HALT || system_state == SYSTEM_POWER_OFF ||
+	    system_state == SYSTEM_RESTART)
+		return -EBUSY;
 
 	/*
 	 * When onlining a CPU, cpu_online_mask is set before kvm_online_cpu()
@@ -5197,6 +5210,8 @@ static int hardware_enable_all(void)
 	cpus_read_lock();
 	mutex_lock(&kvm_lock);
 
+	r = 0;
+
 	kvm_usage_count++;
 	if (kvm_usage_count == 1) {
 		on_each_cpu(hardware_enable_nolock, &failed, 1);