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

[Quentin Perret <qperret@google.com>]

On Friday 27 Nov 2020 at 17:14:11 (+0000), Marc Zyngier wrote:
> 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@kernel.org>
> > > > Signed-off-by: Will Deacon <will@kernel.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?
> 
> 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.

Could we use the trick we have for arm64_ftr_reg_ctrel0 to speed this
up?

Thanks,
Quentin