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

From: Marc Zyngier <[email-masked]>
Date: Fri, 27 Nov 2020 17:14:11 +0000

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 <[email-masked]>
>> > Signed-off-by: Will Deacon <[email-masked]>
>> > ---
>> >  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...

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