linux-kernel - Re: [PATCH v3 05/22] kvm: x86: Check permitted dynamic xfeatures at KVM_SET

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Date:   Tue, 28 Dec 2021 23:38:52 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jing Liu <jing2.liu@...el.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, pbonzini@...hat.com, corbet@....net,
        shuah@...nel.org, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, guang.zeng@...el.com,
        wei.w.wang@...el.com, yang.zhong@...el.com
Subject: Re: [PATCH v3 05/22] kvm: x86: Check permitted dynamic xfeatures at
 KVM_SET_CPUID2

On Wed, Dec 22, 2021, Jing Liu wrote:
> Guest xstate permissions should be set by userspace VMM before vcpu
> creation. Extend KVM_SET_CPUID2 to verify that every feature reported
> in CPUID[0xD] has proper permission set.

Why?  Nothing in the changelog, code, or comments explains why KVM _needs_ to prevent
userspace from advertising bogus features to the guest.  E.g. the virtual address
width check exists because _KVM_ will do the wrong thing given a width other than 48
or 57, and explicity says as much in a comment.