linux-kernel - Re: [RESEND PATCH 07/13] KVM: x86: Add explicit flag for forced emulation on #UD

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Date:   Fri, 23 Aug 2019 07:44:23 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH 07/13] KVM: x86: Add explicit flag for forced
 emulation on #UD

On Fri, Aug 23, 2019 at 04:47:14PM +0300, Liran Alon wrote:
> 
> 
> > On 23 Aug 2019, at 4:07, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
> > 
> > Add an explicit emulation type for forced #UD emulation and use it to
> > detect that KVM should unconditionally inject a #UD instead of falling
> > into its standard emulation failure handling.
> > 
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> The name "forced emulation on #UD" is not clear to me.
> 
> If I understand correctly, EMULTYPE_TRAP_UD is currently used to indicate
> that in case the x86 emulator fails to decode instruction, the caller would
> like the x86 emulator to fail early such that it can handle this condition
> properly.  Thus, I would rename it EMULTYPE_TRAP_DECODE_FAILURE.

EMULTYPE_TRAP_UD is used when KVM intercepts a #UD from hardware.  KVM
only emulates select instructions in this case in order to minmize the
emulator attack surface, e.g.:

	if (unlikely(ctxt->ud) && likely(!(ctxt->d & EmulateOnUD)))
		return EMULATION_FAILED;

To enable testing of the emulator, KVM recognizes a special "opcode" that
triggers full emulation on #UD, e.g. ctxt->ud is false when the #UD was
triggered with the magic prefix.  The prefix is only recognized when the
module param force_emulation_prefix is toggled on, hence the name
EMULTYPE_TRAP_UD_FORCED.

> But this new flag seems to do the same. So I’m left confused.  I’m probably
> missing something trivial here.