Re: [PATCH v12 07/11] KVM: arm64: Use arm64_ftr_bits to sanitise ID register writes

[Oliver Upton <oliver.upton@linux.dev>]

Hey Marc,

On Thu, Jun 15, 2023 at 01:38:34PM +0100, Marc Zyngier wrote:
> > @@ -1528,11 +1613,31 @@ static int get_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> >  static int set_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> >  		      u64 val)
> >  {
> > -	/* This is what we mean by invariant: you can't change it. */
> > -	if (val != read_id_reg(vcpu, rd))
> > -		return -EINVAL;
> > +	u32 id = reg_to_encoding(rd);
> > +	int ret;
> >  
> > -	return 0;
> > +	mutex_lock(&vcpu->kvm->arch.config_lock);
> > +
> > +	/*
> > +	 * Once the VM has started the ID registers are immutable. Reject any
> > +	 * write that does not match the final register value.
> > +	 */
> > +	if (kvm_vm_has_ran_once(vcpu->kvm)) {
> > +		if (val != read_id_reg(vcpu, rd))
> > +			ret = -EBUSY;
> > +		else
> > +			ret = 0;
> > +
> > +		mutex_unlock(&vcpu->kvm->arch.config_lock);
> > +		return ret;
> > +	}
> > +
> > +	ret = arm64_check_features(vcpu, rd, val);
> > +	if (!ret)
> > +		IDREG(vcpu->kvm, id) = val;
> > +
> > +	mutex_unlock(&vcpu->kvm->arch.config_lock);
> > +	return ret;
> 
> ... we now end-up with a *new* error code that userspace was never
> able to see so far.
> 
> This may not be a big deal, but I'd rather err on the side of caution
> by keeping the current, slightly less precise error code.

I completely agree, thanks for catching this. There's already enough
deliberate (theorectical) breakage brought about by this series, want to
avoid any unintended fallout :)

I plan on taking this, and I'll apply a fix on top to dumb down the
return.

-- 
Thanks,
Oliver