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

[Marc Zyngier]

On 2020-11-27 11:53, Will Deacon wrote:
> 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@xxxxxxxxxx>
> > > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > > ---
> > >  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?

Yeah, the sanitized read feels better, if only because that is
what we are going to read in all the valid cases, unfortunately.
read_sanitised_ftr_reg() is sadly not designed to be called on
a fast path, meaning that 32bit guests will do a bsearch() on
the ID-regs every time they exit...

I guess we will have to evaluate how much we loose with this.

Thanks,

        M.
--
Jazz is not dead. It just smells funny...