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

[Halil Pasic]

On Wed, 19 May 2021 11:39:21 -0400
Tony Krowiak <akrowiak@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> ---
>  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;