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x86/reboot: Force all cpus to exit VMX root if VMX is supported

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Force all CPUs to do VMXOFF (via NMI shootdown) during an emergency
reboot if VMX is _supported_, as VMX being off on the current CPU does
not prevent other CPUs from being in VMX root (post-VMXON).  This fixes
a bug where a crash/panic reboot could leave other CPUs in VMX root and
prevent them from being woken via INIT-SIPI-SIPI in the new kernel.

Fixes: d176720d34 ("x86: disable VMX on all CPUs on reboot")
Cc: stable@vger.kernel.org
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David P. Reed <dpreed@deepplum.com>
[sean: reworked changelog and further tweaked comment]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20201231002702.2223707-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2020-12-30 16:26:55 -08:00 committed by Paolo Bonzini
parent aec511ad15
commit ed72736183
1 changed files with 10 additions and 20 deletions
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30
arch/x86/kernel/reboot.c
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@ -538,31 +538,21 @@ static void emergency_vmx_disable_all(void)
local_irq_disable();
/*
* We need to disable VMX on all CPUs before rebooting, otherwise
* we risk hanging up the machine, because the CPU ignores INIT
* signals when VMX is enabled.
* Disable VMX on all CPUs before rebooting, otherwise we risk hanging
* the machine, because the CPU blocks INIT when it's in VMX root.
*
* We can't take any locks and we may be on an inconsistent
* state, so we use NMIs as IPIs to tell the other CPUs to disable
* VMX and halt.
* 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.
*
* For safety, we will avoid running the nmi_shootdown_cpus()
* stuff unnecessarily, but we don't have a way to check
* if other CPUs have VMX enabled. So we will call it only if the
* CPU we are running on has VMX enabled.
*
* We will miss cases where VMX is not enabled on all CPUs. This
* shouldn't do much harm because KVM always enable VMX on all
* CPUs anyway. But we can miss it on the small window where KVM
* is still enabling VMX.
* 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.
*/
if (cpu_has_vmx() && cpu_vmx_enabled()) {
/* Disable VMX on this CPU. */
cpu_vmxoff();
if (cpu_has_vmx()) {
/* Safely force _this_ CPU out of VMX root operation. */
__cpu_emergency_vmxoff();
/* Halt and disable VMX on the other CPUs */
/* Halt and exit VMX root operation on the other CPUs. */
nmi_shootdown_cpus(vmxoff_nmi);
}
}