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From:   Jim Mattson <jmattson@google.com>
Date:   Fri, 10 Dec 2021 19:48:27 -0800
Message-ID: <CALMp9eSLU1kfffC3Du58L8iPY6LmKyVO0yU7c3wEnJAD9JZw4w@mail.gmail.com>
Subject: Re: [PATCH] KVM: x86/svm: Add module param to control PMU virtualization
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Like Xu <like.xu.linux@gmail.com>,
        Maxim Levitsky <mlevitsk@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On Fri, Dec 10, 2021 at 6:15 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 12/10/21 20:25, Jim Mattson wrote:
> > In the long run, I'd like to be able to override this system-wide
> > setting on a per-VM basis, for VMs that I trust. (Of course, this
> > implies that I trust the userspace process as well.)
> >
> > How would you feel if we were to add a kvm ioctl to override this
> > setting, for a particular VM, guarded by an appropriate permissions
> > check, like capable(CAP_SYS_ADMIN) or capable(CAP_SYS_MODULE)?
>
> What's the rationale for guarding this with a capability check?  IIRC
> you don't have such checks for perf_event_open (apart for getting kernel
> addresses, which is not a problem for virtualization).

My reasoning was simply that for userspace to override a mode 0444
kernel module parameter, it should have the rights to reload the
module with the parameter override. I wasn't thinking specifically
about PMU capabilities.