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

[Tony Krowiak]

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;
 };
 
 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)) {
+			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);
+	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;
 	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);
+	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;
-- 
2.30.2