linux-kernel - Re: [PATCH v2 05/21] KVM: x86: Disallow writes to immutable feature MSRs after KVM

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Message-ID: <Y+1ecVEQhgEGIqMy@google.com>
Date:   Wed, 15 Feb 2023 14:36:33 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 05/21] KVM: x86: Disallow writes to immutable feature
 MSRs after KVM_RUN

On Tue, Feb 14, 2023, Like Xu wrote:
> On 10/2/2023 8:31 am, Sean Christopherson wrote:
> > @@ -2168,6 +2187,23 @@ static int do_get_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
> >   static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
> >   {
> > +	u64 val;
> > +
> > +	/*
> > +	 * Disallow writes to immutable feature MSRs after KVM_RUN.  KVM does
> > +	 * not support modifying the guest vCPU model on the fly, e.g. changing
> > +	 * the nVMX capabilities while L2 is running is nonsensical.  Ignore
> > +	 * writes of the same value, e.g. to allow userspace to blindly stuff
> > +	 * all MSRs when emulating RESET.
> > +	 */
> > +	if (vcpu->arch.last_vmentry_cpu != -1 &&
> 
> Three concerns on my mind (to help you think more if any):
> - why not using kvm->created_vcpus;

Because this is a vCPU scoped ioctl().

> - how about different vcpu models of the same guest have different
> feature_msr values;

KVM shouldn't care.  If KVM does care, then that's a completely orthogonal bug
that needs to be fixed separately.

> (although they are not altered after the first run, cases (selftests) may be
> needed to
> show that it is dangerous for KVM);
> 
> - the relative time to set "vcpu->arch.last_vmentry_cpu = vcpu->cpu" is
> still too late,
> since part of the guest code (an attack window) has already been executed on first
> run of kvm_x86_vcpu_run() which may run for a long time;

Again, this is a vCPU scoped ioctl.  The task doing KVM_RUN holds vcpu->mutex so
there is no race.