[PATCH 2/2] KVM: Don't enable hardware after a restart/shutdown is initiated

[Marc Zyngier]

On Fri, 10 Mar 2023 22:14:14 +0000,
Sean Christopherson <seanjc at google.com> wrote:
> 
> 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: 8e1c18157d87 ("KVM: VMX: Disable VMX when system shutdown")
> Signed-off-by: Sean Christopherson <seanjc at google.com>
> ---
>  virt/kvm/kvm_main.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 6cdfbb2c641b..b2bf4c105181 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -5182,7 +5182,20 @@ 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;

Since we now seem to be relying on system_state for most things, is
there any use for 'kvm_rebooting' other than the ease of evaluation in
__svm_vcpu_run?

	M.

-- 
Without deviation from the norm, progress is not possible.