Re: [PATCH v3 3/4] KVM: SVM: Fix CPL updates

[Gleb Natapov <gleb@redhat.com>]

On Mon, Feb 06, 2012 at 10:18:35AM +0100, Kevin Wolf wrote:
> Am 05.02.2012 12:16, schrieb Gleb Natapov:
> > On Fri, Feb 03, 2012 at 07:29:24PM +0100, Kevin Wolf wrote:
> >> Keep CPL at 0 in real mode and at 3 in VM86. In protected/long mode, use
> >> RPL rather than DPL of the code segment.
> >>
> >> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> >> ---
> >>  arch/x86/kvm/svm.c |   19 ++++++++++++++++---
> >>  1 files changed, 16 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> >> index 6a977c1..4124a7e 100644
> >> --- a/arch/x86/kvm/svm.c
> >> +++ b/arch/x86/kvm/svm.c
> >> @@ -1263,6 +1263,21 @@ static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> >>  		wrmsrl(host_save_user_msrs[i], svm->host_user_msrs[i]);
> >>  }
> >>  
> >> +static void svm_update_cpl(struct kvm_vcpu *vcpu)
> >> +{
> >> +	struct vcpu_svm *svm = to_svm(vcpu);
> >> +	int cpl;
> >> +
> >> +	if (!is_protmode(vcpu))
> >> +		cpl = 0;
> >> +	else if (svm->vmcb->save.rflags & X86_EFLAGS_VM)
> >> +		cpl = 3;
> >> +	else
> >> +		cpl = svm->vmcb->save.cs.selector & 0x3;
> >> +
> >> +	svm->vmcb->save.cpl = cpl;
> >> +}
> >> +
> > As you probably know already I think cpl should be updated in
> > svm_get_rflags() too. With current patch restoring CS segment
> > register before rflags register after migration may cause cpl
> > to get wrong value for instance.
> 
> I think you mean set_rflags rather than get_rflags?
> 
Uh, yes.

> Patch 4 adds this with the set_rflags emulator callback.
> 
Sorry, missed that, I expected it to be in this patch for some reason.

I think calling svm_update_cpl() from set_rflags() is incorrect though.
svm_update_cpl() checks cr0 so if guest does "mov 1, %cr0; popf" and
popf happens to be emulated it will change cpl to cs&3 which is
incorrect.

--
			Gleb.