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

Halil Pasic
2021-05-19T17:22:04Z

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?

> };
>
> 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?

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).

> + 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.

> + 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;