Re: [PATCH v3 2/2] s390/vfio-ap: control access to PQAP(AQIC) interception handler

Tony Krowiak
2021-05-19T23:14:42Z

On 5/19/21 1:21 PM, Halil Pasic wrote:
> On Wed, 19 May 2021 11:39:21 -0400
> Tony Krowiak <akrowiak@linux.ibm.com> wrote:
>
>> There is currently nothing that controls access to the structure that
>> contains the function pointer to the handler that processes interception of
>> the PQAP(AQIC) instruction. If the mdev is removed while the PQAP(AQIC)
>> instruction is being intercepted, there is a possibility that the function
>> pointer to the handler can get wiped out prior to the attempt to call it.
>>
>> This patch utilizes RCU to synchronize access to the kvm_s390_module_hook
>> structure used to process interception of the PQAP(AQIC) instruction.
>>
>> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
>> ---
>> arch/s390/include/asm/kvm_host.h | 1 +
>> arch/s390/kvm/priv.c | 47 ++++++++++++++++-----------
>> drivers/s390/crypto/vfio_ap_ops.c | 54 ++++++++++++++++++++++++-------
>> 3 files changed, 73 insertions(+), 29 deletions(-)
>>
>> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
>> index 8925f3969478..4987e82d6116 100644
>> --- a/arch/s390/include/asm/kvm_host.h
>> +++ b/arch/s390/include/asm/kvm_host.h
>> @@ -806,6 +806,7 @@ struct kvm_s390_cpu_model {
>> struct kvm_s390_module_hook {
>> int (*hook)(struct kvm_vcpu *vcpu);
>> struct module *owner;
>> + void *data;
> I guess you need this, because you stopped using the member of struct
> ap_mdev_matrix and instead you kzalloc() a new object. Yet I don't
> understand why do you do so?

I did so because the mdev remove callback frees the matrix_mdev
and I thought I could protect against accessing freed storage by
storing the pointer in the pqap_hook structure; however, since we
can't hold the rcu_read_lock while the handle_pqap is executing - due
to wait conditions - this turns out to be a bad idea. I'm not sure at
this point that we can use RCU for this because the freeing of the
matrix_mdev is independent of pqap processing.

>
>> };
>>
>> struct kvm_s390_crypto {
>> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
>> index 9928f785c677..2d330dfbdb61 100644
>> --- a/arch/s390/kvm/priv.c
>> +++ b/arch/s390/kvm/priv.c
>> @@ -610,8 +610,11 @@ static int handle_io_inst(struct kvm_vcpu *vcpu)
>> static int handle_pqap(struct kvm_vcpu *vcpu)
>> {
>> struct ap_queue_status status = {};
>> + struct kvm_s390_module_hook *pqap_module_hook;
>> + int (*pqap_hook)(struct kvm_vcpu *vcpu);
>> + struct module *owner;
>> unsigned long reg0;
>> - int ret;
>> + int ret = 0;
>> uint8_t fc;
>>
>> /* Verify that the AP instruction are available */
>> @@ -657,24 +660,32 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>> * Verify that the hook callback is registered, lock the owner
>> * and call the hook.
>> */
>> - if (vcpu->kvm->arch.crypto.pqap_hook) {
>> - if (!try_module_get(vcpu->kvm->arch.crypto.pqap_hook->owner))
>> - return -EOPNOTSUPP;
>> - ret = vcpu->kvm->arch.crypto.pqap_hook->hook(vcpu);
>> - module_put(vcpu->kvm->arch.crypto.pqap_hook->owner);
>> - if (!ret && vcpu->run->s.regs.gprs[1] & 0x00ff0000)
>> - kvm_s390_set_psw_cc(vcpu, 3);
>> - return ret;
>> + rcu_read_lock();
>> + pqap_module_hook = rcu_dereference(vcpu->kvm->arch.crypto.pqap_hook);
>> + if (pqap_module_hook) {
>> + pqap_hook = pqap_module_hook->hook;
>> + owner = pqap_module_hook->owner;
>> + rcu_read_unlock();
>> + if (!try_module_get(owner)) {
> Why do this outside the rcu_read lock?

It should be done inside the lock.

>
> What guarantees that the module ain't gone by this time? I don't think
> try_module_get() is guaranteed to give you false if passed in a pointer
> that points to some memory that ain't a struct module any more
> (use-after-free).

That needs to be inside the lock.

>
>> + ret = -EOPNOTSUPP;
>> + } else {
>> + ret = pqap_hook(vcpu);
>> + module_put(owner);
>> + if (!ret && vcpu->run->s.regs.gprs[1] & 0x00ff0000)
>> + kvm_s390_set_psw_cc(vcpu, 3);
>> + }
>> + } else {
>> + rcu_read_unlock();
>> + /*
>> + * A vfio_driver must register a hook.
>> + * No hook means no driver to enable the SIE CRYCB and no
>> + * queues. We send this response to the guest.
>> + */
>> + status.response_code = 0x01;
>> + memcpy(&vcpu->run->s.regs.gprs[1], &status, sizeof(status));
>> + kvm_s390_set_psw_cc(vcpu, 3);
>> }
>> - /*
>> - * A vfio_driver must register a hook.
>> - * No hook means no driver to enable the SIE CRYCB and no queues.
>> - * We send this response to the guest.
>> - */
>> - status.response_code = 0x01;
>> - memcpy(&vcpu->run->s.regs.gprs[1], &status, sizeof(status));
>> - kvm_s390_set_psw_cc(vcpu, 3);
>> - return 0;
>> + return ret;
>> }
>>
>> static int handle_stfl(struct kvm_vcpu *vcpu)
>> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
>> index f90c9103dac2..a6aa3f753ac4 100644
>> --- a/drivers/s390/crypto/vfio_ap_ops.c
>> +++ b/drivers/s390/crypto/vfio_ap_ops.c
>> @@ -16,6 +16,7 @@
>> #include <linux/bitops.h>
>> #include <linux/kvm_host.h>
>> #include <linux/module.h>
>> +#include <linux/rcupdate.h>
>> #include <asm/kvm.h>
>> #include <asm/zcrypt.h>
>>
>> @@ -279,6 +280,7 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>> uint64_t status;
>> uint16_t apqn;
>> struct vfio_ap_queue *q;
>> + struct kvm_s390_module_hook *pqap_module_hook;
>> struct ap_queue_status qstatus = {
>> .response_code = AP_RESPONSE_Q_NOT_AVAIL, };
>> struct ap_matrix_mdev *matrix_mdev;
>> @@ -287,13 +289,17 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>> if (!(vcpu->arch.sie_block->eca & ECA_AIV))
>> return -EOPNOTSUPP;
>>
>> - apqn = vcpu->run->s.regs.gprs[0] & 0xffff;
>> - mutex_lock(&matrix_dev->lock);
>> + rcu_read_lock();
>> + pqap_module_hook = rcu_dereference(vcpu->kvm->arch.crypto.pqap_hook);
>> + if (!pqap_module_hook) {
>> + rcu_read_unlock();
>> + goto set_status;
>> + }
>>
>> - if (!vcpu->kvm->arch.crypto.pqap_hook)
>> - goto out_unlock;
>> - matrix_mdev = container_of(vcpu->kvm->arch.crypto.pqap_hook,
>> - struct ap_matrix_mdev, pqap_hook);
>> + matrix_mdev = pqap_module_hook->data;
>> + rcu_read_unlock();
>> + mutex_lock(&matrix_dev->lock);
> I agree with Jason's assessment. At this point the matrix_dev pointer
> may point to garbage.
>
> Above, I think we can use the pqap_hook function pointer local to
> handle_pqap, because we know that as long as the module is there
> the callback will sit at the same address and won't go away. And
> we do the try_module_get() to ensure that the module stays loaded.

I'll take your word that is true. If so, then I think I can replace
the way we get the matrix_mdev in the pqap handler which
can be done under the matrix_dev->lock. I can write a function
to get the matrix from the list of mdevs in matrix_dev. If the
matrix_mdev has been removed, then we can bail out of the
handle_pqap function. Maybe the RCU can be used after all.

>
>
>> + apqn = vcpu->run->s.regs.gprs[0] & 0xffff;
>>
>> /*
>> * If the KVM pointer is in the process of being set, wait until the
>> @@ -322,9 +328,10 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>> qstatus = vfio_ap_irq_disable(q);
>>
>> out_unlock:
>> + mutex_unlock(&matrix_dev->lock);
>> +set_status:
>> memcpy(&vcpu->run->s.regs.gprs[1], &qstatus, sizeof(qstatus));
>> vcpu->run->s.regs.gprs[1] >>= 32;
>> - mutex_unlock(&matrix_dev->lock);
>> return 0;
>> }
>>
>> @@ -353,8 +360,6 @@ static int vfio_ap_mdev_create(struct mdev_device *mdev)
>> vfio_ap_matrix_init(&matrix_dev->info, &matrix_mdev->matrix);
>> init_waitqueue_head(&matrix_mdev->wait_for_kvm);
>> mdev_set_drvdata(mdev, matrix_mdev);
>> - matrix_mdev->pqap_hook.hook = handle_pqap;
>> - matrix_mdev->pqap_hook.owner = THIS_MODULE;
> I guess the member of struct ap_matrix_mdev is still around, it will
> remain all zero. Is this somehow intentional?
>
>
>> mutex_lock(&matrix_dev->lock);
>> list_add(&matrix_mdev->node, &matrix_dev->mdev_list);
>> mutex_unlock(&matrix_dev->lock);
>> @@ -1085,6 +1090,22 @@ static const struct attribute_group *vfio_ap_mdev_attr_groups[] = {
>> NULL
>> };
>>
>> +static int vfio_ap_mdev_set_pqap_hook(struct ap_matrix_mdev *matrix_mdev,
>> + struct kvm *kvm)
>> +{
>> + struct kvm_s390_module_hook *pqap_hook;
>> +
>> + pqap_hook = kmalloc(sizeof(*kvm->arch.crypto.pqap_hook), GFP_KERNEL);
> What is the extra allocation supposed to buy us?
>
>> + if (!pqap_hook)
>> + return -ENOMEM;
>> + pqap_hook->data = matrix_mdev;
>> + pqap_hook->hook = handle_pqap;
>> + pqap_hook->owner = THIS_MODULE;
>> + rcu_assign_pointer(kvm->arch.crypto.pqap_hook, pqap_hook);
>> +
>> + return 0;
>> +}
>> +
>> /**
>> * vfio_ap_mdev_set_kvm
>> *
>> @@ -1107,6 +1128,7 @@ static const struct attribute_group *vfio_ap_mdev_attr_groups[] = {
>> static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>> struct kvm *kvm)
>> {
>> + int ret;
>> struct ap_matrix_mdev *m;
>>
>> if (kvm->arch.crypto.crycbd) {
>> @@ -1115,6 +1137,10 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>> return -EPERM;
>> }
>>
>> + ret = vfio_ap_mdev_set_pqap_hook(matrix_mdev, kvm);
>> + if (ret)
>> + return ret;
>> +
>> kvm_get_kvm(kvm);
>> matrix_mdev->kvm_busy = true;
>> mutex_unlock(&matrix_dev->lock);
>> @@ -1123,7 +1149,6 @@ static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
>> matrix_mdev->matrix.aqm,
>> matrix_mdev->matrix.adm);
>> mutex_lock(&matrix_dev->lock);
>> - kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
>> matrix_mdev->kvm = kvm;
>> matrix_mdev->kvm_busy = false;
>> wake_up_all(&matrix_mdev->wait_for_kvm);
>> @@ -1161,6 +1186,13 @@ static int vfio_ap_mdev_iommu_notifier(struct notifier_block *nb,
>> return NOTIFY_DONE;
>> }
>>
>> +static void vfio_ap_mdev_unset_pqap_hook(struct kvm *kvm)
>> +{
>> + rcu_assign_pointer(kvm->arch.crypto.pqap_hook, NULL);
>> + synchronize_rcu();
>> + kfree(kvm->arch.crypto.pqap_hook);
>> +}
>> +
>> /**
>> * vfio_ap_mdev_unset_kvm
>> *
>> @@ -1189,11 +1221,11 @@ static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
>>
>> if (matrix_mdev->kvm) {
>> matrix_mdev->kvm_busy = true;
>> + vfio_ap_mdev_unset_pqap_hook(matrix_mdev->kvm);
>> mutex_unlock(&matrix_dev->lock);
>> kvm_arch_crypto_clear_masks(matrix_mdev->kvm);
>> mutex_lock(&matrix_dev->lock);
>> vfio_ap_mdev_reset_queues(matrix_mdev->mdev);
>> - matrix_mdev->kvm->arch.crypto.pqap_hook = NULL;
>> kvm_put_kvm(matrix_mdev->kvm);
>> matrix_mdev->kvm = NULL;
>> matrix_mdev->kvm_busy = false;