linux-kernel - Re: [PATCH] KVM: x86: emulate reserved nops from 0f/18 to 0f/1f

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Date:   Tue, 19 May 2020 09:43:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: emulate reserved nops from 0f/18 to 0f/1f

On 19/05/20 08:02, Sean Christopherson wrote:
> On Mon, May 18, 2020 at 07:37:08PM +0200, Paolo Bonzini wrote:
>> On 18/05/20 18:07, Sean Christopherson wrote:
>>> On Fri, May 15, 2020 at 12:19:19PM -0400, Paolo Bonzini wrote:
>>>> Instructions starting with 0f18 up to 0f1f are reserved nops, except those
>>>> that were assigned to MPX.
>>> Well, they're probably reserved NOPs again :-D.
>>
>> So are you suggesting adding them back to the list as well?
> 
> Doesn't KVM still support MPX?
> 
>>>> These include the endbr markers used by CET.
>>> And RDSPP.  Wouldn't it make sense to treat RDSPP as a #UD even though it's
>>> a NOP if CET is disabled?  The logic being that a sane guest will execute
>>> RDSSP iff CET is enabled, and in that case it'd be better to inject a #UD
>>> than to silently break the guest.
>>
>> We cannot assume that guests will bother checking CPUID before invoking
>> RDSPP.  This is especially true userspace, which needs to check if CET
>> is enable for itself and can only use RDSPP to do so.
> 
> Ugh, yeah, just read through the CET enabling thread that showed code snippets
> that do exactly this.
> 
> I assume it would be best to make SHSTK dependent on unrestricted guest?
> Emulating RDSPP by reading vmcs.GUEST_SSP seems pointless as it will become
> statle apart on the first emulated CALL/RET.

Running arbitrary code under the emulator is problematic anyway with
CET, since you won't be checking ENDBR markers or updating the state
machine.  So perhaps in addition to what you say we should have a mode
where, unless unrestricted guest is disabled, the emulator only accepts
I/O, MOV and ALU instructions.

Paolo