linux-kernel - Re: [PATCH v4 03/14] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched EL0 support

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Date:   Fri, 27 Nov 2020 11:53:05 +0000
From:   Will Deacon <will@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH v4 03/14] KVM: arm64: Kill 32-bit vCPUs on systems with
 mismatched EL0 support

On Fri, Nov 27, 2020 at 10:26:47AM +0000, Marc Zyngier wrote:
> On 2020-11-24 15:50, Will Deacon wrote:
> > If a vCPU is caught running 32-bit code on a system with mismatched
> > support at EL0, then we should kill it.
> > 
> > Acked-by: Marc Zyngier <maz@...nel.org>
> > Signed-off-by: Will Deacon <will@...nel.org>
> > ---
> >  arch/arm64/kvm/arm.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 5750ec34960e..d322ac0f4a8e 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -633,6 +633,15 @@ static void check_vcpu_requests(struct kvm_vcpu
> > *vcpu)
> >  	}
> >  }
> > 
> > +static bool vcpu_mode_is_bad_32bit(struct kvm_vcpu *vcpu)
> > +{
> > +	if (likely(!vcpu_mode_is_32bit(vcpu)))
> > +		return false;
> > +
> > +	return !system_supports_32bit_el0() ||
> > +		static_branch_unlikely(&arm64_mismatched_32bit_el0);
> > +}
> > +
> >  /**
> >   * kvm_arch_vcpu_ioctl_run - the main VCPU run function to execute
> > guest code
> >   * @vcpu:	The VCPU pointer
> > @@ -816,7 +825,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> >  		 * with the asymmetric AArch32 case), return to userspace with
> >  		 * a fatal error.
> >  		 */
> > -		if (!system_supports_32bit_el0() && vcpu_mode_is_32bit(vcpu)) {
> > +		if (vcpu_mode_is_bad_32bit(vcpu)) {
> >  			/*
> >  			 * As we have caught the guest red-handed, decide that
> >  			 * it isn't fit for purpose anymore by making the vcpu
> 
> Given the new definition of system_supports_32bit_el0() in the previous
> patch,
> why do we need this patch at all?

I think the check is still needed, as this is an unusual case where we
want to reject the mismatched system. For example, imagine
'arm64_mismatched_32bit_el0' is true and we're on a mismatched system: in
this case system_supports_32bit_el0() will return 'true' because we
allow 32-bit applications to run, we support the 32-bit personality etc.

However, we still want to terminate 32-bit vCPUs if we spot them in this
situation, so we have to check for:

	!system_supports_32bit_el0() ||
	static_branch_unlikely(&arm64_mismatched_32bit_el0)

so that we only allow 32-bit vCPUs when all of the physical CPUs support
it at EL0.

I could make this clearer either by adding a comment, or avoiding
system_supports_32bit_el0() entirely here and just checking the
sanitised SYS_ID_AA64PFR0_EL1 register directly instead.

What do you prefer?

Will