Re: [PATCH 2/2] x86/reboot: Disable virtualization in an emergency if SVM is supported

Vitaly Kuznetsov
2022-05-12T08:38:26Z

Sean Christopherson <seanjc@google.com> writes:

> Disable SVM on all CPUs via NMI shootdown during an emergency reboot.
> Like VMX, SVM can block INIT and thus prevent bringing up other CPUs via
> INIT-SIPI-SIPI.
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kernel/reboot.c | 26 ++++++++++++++------------
> 1 file changed, 14 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> index f9543a4e9b09..33c1f4883b27 100644
> --- a/arch/x86/kernel/reboot.c
> +++ b/arch/x86/kernel/reboot.c
> @@ -533,27 +533,29 @@ static void nmi_shootdown_nop(int cpu, struct pt_regs *regs)
> /* Nothing to do, the NMI shootdown handler disables virtualization. */
> }
>
> -/* Use NMIs as IPIs to tell all CPUs to disable virtualization */
> -static void emergency_vmx_disable_all(void)
> +static void emergency_reboot_disable_virtualization(void)
> {
> /* Just make sure we won't change CPUs while doing this */
> local_irq_disable();
>
> /*
> - * Disable VMX on all CPUs before rebooting, otherwise we risk hanging
> - * the machine, because the CPU blocks INIT when it's in VMX root.
> + * Disable virtualization on all CPUs before rebooting to avoid hanging
> + * the system, as VMX and SVM block INIT when running in the host
> *
> * We can't take any locks and we may be on an inconsistent state, so
> - * use NMIs as IPIs to tell the other CPUs to exit VMX root and halt.
> + * use NMIs as IPIs to tell the other CPUs to disable VMX/SVM and halt.
> *
> - * Do the NMI shootdown even if VMX if off on _this_ CPU, as that
> - * doesn't prevent a different CPU from being in VMX root operation.
> + * Do the NMI shootdown even if virtualization is off on _this_ CPU, as
> + * other CPUs may have virtualization enabled.
> */
> - if (cpu_has_vmx()) {
> - /* Safely force _this_ CPU out of VMX root operation. */
> - __cpu_emergency_vmxoff();
> + if (cpu_has_vmx() || cpu_has_svm(NULL)) {
> + /* Safely force _this_ CPU out of VMX/SVM operation. */
> + if (cpu_has_vmx())
> + __cpu_emergency_vmxoff();
> + else
> + cpu_emergency_svm_disable();
>
> - /* Halt and exit VMX root operation on the other CPUs. */
> + /* Disable VMX/SVM and halt on other CPUs. */
> nmi_shootdown_cpus(nmi_shootdown_nop);
> }

Nitpicking: we can get rid of the second cpu_has_vmx() if we write this
as:

if (cpu_has_vmx())
__cpu_emergency_vmxoff();
else if (cpu_has_svm(NULL))
cpu_emergency_svm_disable();
else
return;

nmi_shootdown_cpus(nmi_shootdown_nop);

> }
> @@ -590,7 +592,7 @@ static void native_machine_emergency_restart(void)
> unsigned short mode;
>
> if (reboot_emergency)
> - emergency_vmx_disable_all();
> + emergency_reboot_disable_virtualization();
>
> tboot_shutdown(TB_SHUTDOWN_REBOOT);

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

--
Vitaly