linux-kernel - Re: [PATCH] KVM: fix sil/dil/bpl/spl in the mod/rm fields

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Message-ID: <51ACCB43.50803@redhat.com>
Date:	Mon, 03 Jun 2013 18:58:43 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Avi Kivity <avi.kivity@...il.com>, kvm <kvm@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: fix sil/dil/bpl/spl in the mod/rm fields

Il 03/06/2013 18:40, Gleb Natapov ha scritto:
>> > Won't work, vmx won't let you enter in such a configuration.
> 
> Why? It is possible to have NULL descriptor in 32bit mode with vmx. But
> we do not usually intercept #GP while executing 32bit mode, so we will
> have to track if there is artificial NULL selector and enables #GP
> interception and then emulate on every #GP.

Yes, that's what I had in mind.  Of course for invalid CS you do have to
emulate.

>> > Maybe you can detect the exact code sequence (%eip, some instructions,
>> > register state) and clear %fs and %gs.
> My be we can set dpl to rpl unconditionally on a switch from 16 to 32
> bit. The only problem I can see with it is that if a guest enters user
> mode without explicitly reload the segment it will be accessible by a
> user mode code, but I am not sure it is well defined what dpl of a 16
> bit segment is after transition to 32 bit mode anyway, so it would be
> crazy to do so.

That too, or just set it to 3.  But perhaps the #GP interception
wouldn't be too hard.

Paolo
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