linux-kernel - Re: [PATCH] KVM/x86: pmu: Fix #GP condition check for RDPMC emulation

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Date:   Wed, 8 Jul 2020 17:54:41 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Like Xu <like.xu@...ux.intel.com>, kvm@...r.kernel.org,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM/x86: pmu: Fix #GP condition check for RDPMC emulation

On 08/07/20 17:45, Sean Christopherson wrote:
> On Wed, Jul 08, 2020 at 05:31:14PM +0200, Paolo Bonzini wrote:
>> The order follows the SDM.  I'm tempted to remove the CR0 check
>> altogether, since non-protected-mode always runs at CPL0 AFAIK, but let's
>> keep it close to what the manual says.
> 
> Heh, it wouldn't surprise me in the least if there's a way to get the SS
> arbyte to hold a non-zero DPL in real mode :-).

I'm not sure if SMM lets you set non-zero SS.DPL in real mode.  It's one
of the few things that are checked with unrestricted guest mode so
there's hope; on the other hand I know for sure that in the past RSM
could get you to VM86 mode with CPL=0, while in VMX it causes vmentry to
fail.

It would be an interesting testcase to write for KVM, to see if you get
a vmentry failure after you set the hidden AR bytes that way and RSM...

Paolo