linux-kernel - Re: [RFC PATCH v3 01/11] KVM: Capture VM start

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Date:   Wed, 12 Jan 2022 10:29:07 -0800
From:   Raghavendra Rao Ananta <rananta@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Reiji Watanabe <reijiw@...gle.com>, Marc Zyngier <maz@...nel.org>,
        Andrew Jones <drjones@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Peter Shier <pshier@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [RFC PATCH v3 01/11] KVM: Capture VM start

On Tue, Jan 11, 2022 at 11:16 AM Jim Mattson <jmattson@...gle.com> wrote:
>
> On Tue, Jan 11, 2022 at 10:52 AM Raghavendra Rao Ananta
> <rananta@...gle.com> wrote:
> >
> > On Mon, Jan 10, 2022 at 3:57 PM Jim Mattson <jmattson@...gle.com> wrote:
> > >
> > > On Mon, Jan 10, 2022 at 3:07 PM Raghavendra Rao Ananta
> > > <rananta@...gle.com> wrote:
> > > >
> > > > On Fri, Jan 7, 2022 at 4:05 PM Jim Mattson <jmattson@...gle.com> wrote:
> > > > >
> > > > > On Fri, Jan 7, 2022 at 3:43 PM Raghavendra Rao Ananta
> > > > > <rananta@...gle.com> wrote:
> > > > > >
> > > > > > Hi Reiji,
> > > > > >
> > > > > > On Thu, Jan 6, 2022 at 10:07 PM Reiji Watanabe <reijiw@...gle.com> wrote:
> > > > > > >
> > > > > > > Hi Raghu,
> > > > > > >
> > > > > > > On Tue, Jan 4, 2022 at 11:49 AM Raghavendra Rao Ananta
> > > > > > > <rananta@...gle.com> wrote:
> > > > > > > >
> > > > > > > > Capture the start of the KVM VM, which is basically the
> > > > > > > > start of any vCPU run. This state of the VM is helpful
> > > > > > > > in the upcoming patches to prevent user-space from
> > > > > > > > configuring certain VM features after the VM has started
> > > > > > > > running.
> > > > >
> > > > > What about live migration, where the VM has already technically been
> > > > > started before the first call to KVM_RUN?
> > > >
> > > > My understanding is that a new 'struct kvm' is created on the target
> > > > machine and this flag should be reset, which would allow the VMM to
> > > > restore the firmware registers. However, we would be running KVM_RUN
> > > > for the first time on the target machine, thus setting the flag.
> > > > So, you are right; It's more of a resume operation from the guest's
> > > > point of view. I guess the name of the variable is what's confusing
> > > > here.
> > >
> > > I was actually thinking that live migration gives userspace an easy
> > > way to circumvent your restriction. You said, "This state of the VM is
> > > helpful in the upcoming patches to prevent user-space from configuring
> > > certain VM features after the VM has started running." However, if you
> > > don't ensure that these VM features are configured the same way on the
> > > target machine as they were on the source machine, you have not
> > > actually accomplished your stated goal.
> > >
> > Isn't that up to the VMM to save/restore and validate the registers
> > across migrations?
>
> Yes, just as it is up to userspace not to make bad configuration
> changes after the first VMRUN.
>
> > Perhaps I have to re-word my intentions for the patch- userspace
> > should be able to configure the registers before issuing the first
> > KVM_RUN.
>
> Perhaps it would help if you explained *why* you are doing this. It
> sounds like you are either trying to protect against a malicious
> userspace, or you are trying to keep userspace from doing something
> stupid. In general, kvm only enforces constraints that are necessary
> to protect the host. If that's what you're doing, I don't understand
> why live migration doesn't provide an end-run around your protections.
It's mainly to safeguard the guests. With respect to migration, KVM
and the userspace are collectively playing a role here. It's up to the
userspace to ensure that the registers are configured the same across
migrations and KVM ensures that the userspace doesn't modify the
registers after KVM_RUN so that they don't see features turned OFF/ON
during execution. I'm not sure if it falls into the definition of
protecting the host. Do you see a value in adding this extra
protection from KVM?

Regards,
Raghavendra