linux-kernel - Re: [PATCH 08/14] kvm: x86: don't kill guest on unknown exit reason

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Date:	Fri, 24 Oct 2014 15:26:10 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH 08/14] kvm: x86: don't kill guest on unknown exit reason

On Fri, Oct 24, 2014 at 2:54 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> On 10/24/2014 07:57 PM, Andy Lutomirski wrote:
>> > KVM_EXIT_UNKNOWN is a kvm bug, we don't really know whether it was
>> > triggered by a priveledged application.  Let's not kill the guest: WARN
>> > and inject #UD instead.
>>
>> This scares me a bit.  For guest CPL3, it's probably okay.  For guest
>> CPL0, on the other hand, #UD might not use IST (or a task switch on
>> 32-bit guests), resulting in possible corruption if unprivileged code
>> controls SP.  Admittedly, there aren't that many contexts from which
>> that should happen (on Linux, at least), but something like #DF (or even
>> a triple fault) might be safer if the guest is at CPL0 when this happens.
>
> This in practice will only happen for VMX instructions (INVVPID in this
> patch set, INVEPT on some older kernels); all other intercepts can be
> turned on or off at will.
>
> For unknown exits we will not have exposed those instructions in the VMX
> capabilities (or perhaps we will not have exposed VMX at all in CPUID on
> the older kernels).  So #UD is the right thing to do.
>

Fair enough.

--Andy
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