lkml.org 
[lkml]   [2022]   [Jan]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0
Date
> From: Paolo Bonzini <pbonzini@redhat.com>
> Sent: Tuesday, January 25, 2022 12:30 AM
>
> 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@gmail.com>
> >>>> Sent: Sunday, January 23, 2022 1:50 PM
> >>>>
> >>>> From: Like Xu <likexu@tencent.com>
> >>>>
> >>>> 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.
>

Would that lead to a weird situation where although KVM says no support
of guest permissions while the user can still request them via prctl()?

I wonder whether it's cleaner to do it still via prctl() if we really want to
enhance this part. But as you said then it needs a mechanism to know
KVM's supported_xcr0 (and if KVM is not loaded then no guest permission
support at all)...

Thanks
Kevin
\
 
 \ /
  Last update: 2022-01-25 04:39    [W:0.101 / U:0.452 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site