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

[Paolo Bonzini]

On 1/24/22 17:23, Sean Christopherson wrote:
> On Mon, Jan 24, 2022, Like Xu wrote:
> > 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:
>
> There are no inconsistencies, at least not in the examples below, as the examples
> are intended to work in host context.  guest_supported_xcr0 is about what the guest
> is/isn't allowed to access, it has no bearing on what host userspace can/can't do.
> Or are you talking about a different type of inconsistency?

The extra complication is that arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM) 
changes what host userspace can/can't do.  It would be easier if we 
could just say that KVM_GET_SUPPORTED_CPUID returns "the most" that 
userspace can do, but we already have the contract that userspace can
take KVM_GET_SUPPORTED_CPUID and pass it straight to KVM_SET_CPUID2.

Therefore,  KVM_GET_SUPPORTED_CPUID must limit its returned values to 
what has already been enabled.

While reviewing the QEMU part of AMX support (this morning), I also 
noticed that there is no equivalent for guest permissions of 
ARCH_GET_XCOMP_SUPP.  This needs to know KVM's supported_xcr0, so it's 
probably best realized as a new KVM_CHECK_EXTENSION rather than as an 
arch_prctl.

Paolo

> > - __do_cpuid_func
>
> Reporting what KVM supports to host userspace.
>
> > - kvm_mpx_supported
>
> This is a check on host support.
>
> > - kvm_vcpu_ioctl_x86_set_xsave
>
> "write" from host userspace.
>
> > Have you identified all their areas of use ?