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

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Date:   Tue, 18 Jan 2022 23:47:13 -0800
From:   Reiji Watanabe <reijiw@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Raghavendra Rao Ananta <rananta@...gle.com>, kvm@...r.kernel.org,
        Marc Zyngier <maz@...nel.org>, Peter Shier <pshier@...gle.com>,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [RFC PATCH v3 01/11] KVM: Capture VM start

On Tue, Jan 18, 2022 at 4:07 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Fri, Jan 14, 2022, Reiji Watanabe wrote:
> > The restriction, with which KVM doesn't need to worry about the changes
> > in the registers after KVM_RUN, could potentially protect or be useful
> > to protect KVM and simplify future changes/maintenance of the KVM codes
> > that consumes the values.
>
> That sort of protection is definitely welcome, the previously mentioned CPUID mess
> on x86 would have benefit greatly by KVM being restrictive in the past.  That said,
> hooking KVM_RUN is likely the wrong way to go about implementing any restrictions.
> Running a vCPU is where much of the vCPU's state is explicitly consumed, but it's
> all too easy for KVM to implicity/indirectly consume state via a different ioctl(),
> e.g. if there are side effects that are visible in other registers, than an update
> can also be visible to userspace via KVM_{G,S}ET_{S,}REGS, at which point disallowing
> modifying state after KVM_RUN but not after reading/writing regs is arbitrary and
> inconsitent.

Thank you for your comments !
I think I understand your concern, and that's a great point.
That's not the case for those pseudo registers though at least for now :)
BTW, is this concern specific to hooking KVM_RUN ? (Wouldn't it be the
same for the option with "if kvm->created_vcpus > 0" ?)


> If possible, preventing modification if kvm->created_vcpus > 0 is ideal as it's
> a relatively common pattern in KVM, and provides a clear boundary to userpace
> regarding what is/isn't allowed.

Yes, I agree that would be better in general.  For (pseudo) registers,
I would think preventing modification if kvm->created_vcpus > 0 might
not be a very good option for KVM/ARM though considering usage of
KVM_GET_REG_LIST and KVM_{G,S}ET_ONE_REG.

Thanks,
Reiji