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

[Tony Krowiak]

On 05/11/2018 12:08 PM, Halil Pasic wrote:
> 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?

vm1 will get the kvm->lock first in your scenario when
kvm_ap_configure_matrix(m1) is invoked. Since the other
functions - i.e., kvm_ap_validate_queue_sharing(m1) and
kvm_ap_set_crycb_masks(m1) - are static and only called
from the kvm_ap_configure_matrix(m1), your scenario
can never happen because vm2 will not get the lock until
kvm_ap_configure_matrix(m1) has completed. I see your
point, however, and maybe I should also acquire the kvm_lock.

> 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);
> > +