Re: [PATCH v2 05/11] KVM: x86: emulator: update the emulation mode after CR0 write

[Sean Christopherson]

On Thu, Jul 21, 2022, Maxim Levitsky wrote:
> On Wed, 2022-07-20 at 23:50 +0000, Sean Christopherson wrote:
> > On Tue, Jun 21, 2022, Maxim Levitsky wrote:
> > > CR0.PE toggles real/protected mode, thus its update
> > > should update the emulation mode.
> > > 
> > > This is likely a benign bug because there is no writeback
> > > of state, other than the RIP increment, and when toggling
> > > CR0.PE, the CPU has to execute code from a very low memory address.
> > > 
> > > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > > ---
> > >  arch/x86/kvm/emulate.c | 13 ++++++++++++-
> > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > > index 6f4632babc4cd8..002687d17f9364 100644
> > > --- a/arch/x86/kvm/emulate.c
> > > +++ b/arch/x86/kvm/emulate.c
> > > @@ -3659,11 +3659,22 @@ static int em_movbe(struct x86_emulate_ctxt *ctxt)
> > >  
> > >  static int em_cr_write(struct x86_emulate_ctxt *ctxt)
> > >  {
> > > -	if (ctxt->ops->set_cr(ctxt, ctxt->modrm_reg, ctxt->src.val))
> > > +	int cr_num = ctxt->modrm_reg;
> > > +	int r;
> > > +
> > > +	if (ctxt->ops->set_cr(ctxt, cr_num, ctxt->src.val))
> > >  		return emulate_gp(ctxt, 0);
> > >  
> > >  	/* Disable writeback. */
> > >  	ctxt->dst.type = OP_NONE;
> > > +
> > > +	if (cr_num == 0) {
> > > +		/* CR0 write might have updated CR0.PE */
> > 
> > Or toggled CR0.PG.  
> 
> I thought about it but paging actually does not affect the CPU mode.

Toggling CR0.PG when EFER.LME=1 (and CR4.PAE=1) switches the CPU in and out of
long mode.  That's why I mentioned the EFER.LMA thing below.  It's also notable
in that the only reason we don't have to handle CR4 here is because clearing
CR4.PAE while long is active causes a #GP.  
 
> E.g if you are in protected mode, instructions execute the same regardless
> if you have paging or not.
> 
> (There are probably some exceptions but you understand what I mean).
> 
> Best regards,
> 	Maxim Levitsky
> 
> > It's probably also worth noting that ->set_cr() handles side
> > effects to other registers, e.g. the lack of an EFER.LMA update makes this look
> > suspicious at first glance.