linux-kernel - Re: [PATCH v5 06/13] KVM: s390: interfaces to manage guest's AP matrix

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Date:   Fri, 11 May 2018 18:08:02 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, kwankhede@...dia.com,
        bjsdjshi@...ux.vnet.ibm.com, pbonzini@...hat.com,
        alex.williamson@...hat.com, pmorel@...ux.vnet.ibm.com,
        alifm@...ux.vnet.ibm.com, mjrosato@...ux.vnet.ibm.com,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com
Subject: Re: [PATCH v5 06/13] KVM: s390: interfaces to manage guest's AP
 matrix



On 05/07/2018 05:11 PM, Tony Krowiak wrote:
> Provides interfaces to manage the AP adapters, usage domains
> and control domains assigned to a KVM guest.
> 
> The guest's SIE state description has a satellite structure called the
> Crypto Control Block (CRYCB) containing three bitmask fields
> identifying the adapters, queues (domains) and control domains
> assigned to the KVM guest:

[..]

> index 00bcfb0..98b53c7 100644
> --- a/arch/s390/kvm/kvm-ap.c
> +++ b/arch/s390/kvm/kvm-ap.c
> @@ -7,6 +7,7 @@

[..]

> +
> +/**
> + * kvm_ap_validate_queue_sharing
> + *
> + * Verifies that the APQNs derived from the cross product of the AP adapter IDs
> + * and AP queue indexes comprising the AP matrix are not configured for
> + * another guest. AP queue sharing is not allowed.
> + *
> + * @kvm: the KVM guest
> + * @matrix: the AP matrix
> + *
> + * Returns 0 if the APQNs are valid, otherwise; returns -EBUSY.
> + */
> +static int kvm_ap_validate_queue_sharing(struct kvm *kvm,
> +					 struct kvm_ap_matrix *matrix)
> +{
> +	struct kvm *vm;
> +	unsigned long *apm, *aqm;
> +	unsigned long apid, apqi;
> +
> +
> +	/* No other VM may share an AP Queue with the input VM */
> +	list_for_each_entry(vm, &vm_list, vm_list) {
> +		if (kvm == vm)
> +			continue;
> +
> +		apm = kvm_ap_get_crycb_apm(vm);
> +		if (!bitmap_and(apm, apm, matrix->apm, matrix->apm_max + 1))
> +			continue;
> +
> +		aqm = kvm_ap_get_crycb_aqm(vm);
> +		if (!bitmap_and(aqm, aqm, matrix->aqm, matrix->aqm_max + 1))
> +			continue;
> +
> +		for_each_set_bit_inv(apid, apm, matrix->apm_max + 1)
> +			for_each_set_bit_inv(apqi, aqm, matrix->aqm_max + 1)
> +				kvm_ap_log_sharing_err(vm, apid, apqi);
> +
> +		return -EBUSY;
> +	}
> +
> +	return 0;
> +}
> +
> +int kvm_ap_configure_matrix(struct kvm *kvm, struct kvm_ap_matrix *matrix)
> +{
> +	int ret = 0;
> +
> +	mutex_lock(&kvm->lock);

You seem to take only kvm->lock, vm_list however (used in
kvm_ap_validate_queue_sharing()) seems to be protected by
kvm_lock.

Can you tell me why is this supposed to be safe?

What is supposed to prevent an execution like
vm1: call kvm_ap_configure_matrix(m1)
vm2: call kvm_ap_configure_matrix(m2)
vm1: call kvm_ap_validate_queue_sharing(m1)
vm2: call kvm_ap_validate_queue_sharing(m2)
vm1: call kvm_ap_set_crycb_masks(m1)
vm2: call kvm_ap_set_crycb_masks(m2)

where, let's say, m1 and m2 are equal in the sense that the
mask values are the same?

Regards,
Halil

> +
> +	ret = kvm_ap_validate_queue_sharing(kvm, matrix);
> +	if (ret)
> +		goto done;
> +
> +	kvm_ap_set_crycb_masks(kvm, matrix);
> +
> +done:
> +	mutex_unlock(&kvm->lock);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL(kvm_ap_configure_matrix);
> +