> From: Paolo Bonzini <pbonzini@xxxxxxxxxx> > 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@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. > 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
