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Date:   Tue, 25 Jan 2022 15:10:52 +0000
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From:   Marc Zyngier <maz@kernel.org>
To:     Sean Christopherson <seanjc@google.com>
Cc:     Raghavendra Rao Ananta <rananta@google.com>,
        Jim Mattson <jmattson@google.com>, kvm@vger.kernel.org,
        Will Deacon <will@kernel.org>, Peter Shier <pshier@google.com>,
        linux-kernel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        kvmarm@lists.cs.columbia.edu,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH v3 01/11] KVM: Capture VM start
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It's probably time I jump on this thread

On Thu, 13 Jan 2022 17:21:19 +0000,
Sean Christopherson <seanjc@google.com> wrote:
> 
> On Wed, Jan 12, 2022, Raghavendra Rao Ananta wrote:
> > On Tue, Jan 11, 2022 at 11:16 AM Jim Mattson <jmattson@google.com> wrote:
> > > 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?
> 
> Short answer: probably not?

Well, I beg to defer.

> There is precedent for disallowing userspace from doing stupid
> things, but that's either for KVM's protection (as Jim pointed out),
> or because KVM can't honor the change, e.g. x86 is currently in the
> process of disallowing most CPUID changes after KVM_RUN because KVM
> itself consumes the CPUID information and KVM doesn't support
> updating some of it's own internal state (because removing features
> like GB hugepage support is nonsensical and would require a large
> pile of complicated, messy code).

We provide quite a lot of CPU emulation based on the feature
registers exposed to the guest. Allowing userspace to change this
stuff once the guest is running is a massive attack vector.

> Restricing CPUID changes does offer some "protection" to the guest, but that's
> not the goal.  E.g. KVM won't detect CPUID misconfiguration in the migration
> case, and trying to do so is a fool's errand.
> 
> If restricting updates in the arm64 is necessary to ensure KVM provides sane
> behavior, then it could be justified.  But if it's purely a sanity check on
> behalf of the guest, then it's not justified.

No. This is about preventing userspace from tripping the kernel into
doing things it really shouldn't by flipping bits of configuration
that should be set in stone.

	M.

-- 
Without deviation from the norm, progress is not possible.