linux-kernel - Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported

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Message-ID: <Ye7SbfPL/QAjOI6s@google.com>
Date:   Mon, 24 Jan 2022 16:23:09 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for
 vcpu->arch.guest_supported_xcr0

On Mon, Jan 24, 2022, Like Xu wrote:
> On 24/1/2022 3:06 pm, Tian, Kevin wrote:
> > > From: Like Xu <like.xu.linux@...il.com>
> > > Sent: Sunday, January 23, 2022 1:50 PM
> > > 
> > > From: Like Xu <likexu@...cent.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?

> - __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 ?