Messages in this thread | | | Date | Mon, 24 Jan 2022 17:29:31 +0100 | Subject | Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 | From | Paolo Bonzini <> |
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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.
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 ? >
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