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

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Date:   Wed, 29 Dec 2021 02:18:28 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Liu, Jing2" <jing2.liu@...el.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "jing2.liu@...ux.intel.com" <jing2.liu@...ux.intel.com>,
        "Zeng, Guang" <guang.zeng@...el.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        "Zhong, Yang" <yang.zhong@...el.com>
Subject: RE: [PATCH v3 05/22] kvm: x86: Check permitted dynamic xfeatures at
 KVM_SET_CPUID2

> From: Sean Christopherson <seanjc@...gle.com>
> Sent: Wednesday, December 29, 2021 7:39 AM
> 
> 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.

Advertising a known bogus feature due to lacking of permission does
no good compared to failing it early even before the guest is running. This
also avoids tons of complexity at run-time to deal with permission
violation (e.g. introducing new kvm exit reason if you tracked the v2
discussion).

But yes, we should add a clear comment here.

Thanks
Kevin