From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>,
Like Xu <like.xu.linux@gmail.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
"Liu, Jing2" <jing2.liu@intel.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Joerg Roedel <joro@8bytes.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"Zhong, Yang" <yang.zhong@intel.com>
Subject: Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0
Date: Mon, 24 Jan 2022 17:29:31 +0100 [thread overview]
Message-ID: <e5744e0b-00fc-8563-edb7-b6bf52c63b0e@redhat.com> (raw)
In-Reply-To: <Ye7SbfPL/QAjOI6s@google.com>
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 ?
>
next prev parent reply other threads:[~2022-01-24 16:29 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-23 5:50 [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 Like Xu
2022-01-24 7:06 ` Tian, Kevin
2022-01-24 7:17 ` Like Xu
2022-01-24 16:23 ` Sean Christopherson
2022-01-24 16:29 ` Paolo Bonzini [this message]
2022-01-25 1:54 ` Tian, Kevin
2022-01-25 8:27 ` Paolo Bonzini
2022-01-26 1:23 ` Tian, Kevin
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