Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0

[Like Xu]

On 24/1/2022 3:06 pm, Tian, Kevin wrote:
> > From: Like Xu <like.xu.linux@xxxxxxxxx>
> > Sent: Sunday, January 23, 2022 1:50 PM
> >
> > From: Like Xu <likexu@xxxxxxxxxxx>
> >
> > A malicious user space can bypass xstate_get_guest_group_perm() in the
> > KVM_GET_SUPPORTED_CPUID mechanism and obtain unpermitted xfeatures,
> > since the validity check of xcr0 depends only on guest_supported_xcr0.
>
> Unpermitted xfeatures cannot pass kvm_check_cpuid()...

Indeed, 5ab2f45bba4894a0db4af8567da3efd6228dd010.

This part of logic is pretty fragile and fragmented due to semantic
inconsistencies between supported_xcr0 and guest_supported_xcr0
in other three places:

- __do_cpuid_func
- kvm_mpx_supported
- kvm_vcpu_ioctl_x86_set_xsave

Have you identified all their areas of use ?

> > Fixes: 445ecdf79be0 ("kvm: x86: Exclude unpermitted xfeatures at
> > KVM_GET_SUPPORTED_CPUID")
> > Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/cpuid.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 3902c28fb6cb..1bd4d560cbdd 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -266,7 +266,8 @@ static void kvm_vcpu_after_set_cpuid(struct
> > kvm_vcpu *vcpu)
> >   		vcpu->arch.guest_supported_xcr0 = 0;
> >   	else
> >   		vcpu->arch.guest_supported_xcr0 =
> > -			(best->eax | ((u64)best->edx << 32)) &
> > supported_xcr0;
> > +			(best->eax | ((u64)best->edx << 32)) &
> > +			(supported_xcr0 & xstate_get_guest_group_perm());
> >
> >   	/*
> >   	 * Bits 127:0 of the allowed SECS.ATTRIBUTES (CPUID.0x12.0x1)
> > enumerate
> > --
> > 2.33.1