[PATCH] vfio: fix deadlock between group lock and kvm lock

[Matthew Rosato]

After 51cdc8bc120e, we have another deadlock scenario between the
kvm->lock and the vfio group_lock with two different codepaths acquiring
the locks in different order.  Specifically in vfio_open_device, vfio
holds the vfio group_lock when issuing device->ops->open_device but some
drivers (like vfio-ap) need to acquire kvm->lock during their open_device
routine;  Meanwhile, kvm_vfio_release will acquire the kvm->lock first
before calling vfio_file_set_kvm which will acquire the vfio group_lock.

To resolve this, let's remove the need for the vfio group_lock from the
kvm_vfio_release codepath.  This is done by introducing a new spinlock to
protect modifications to the vfio group kvm pointer, and acquiring a kvm
ref from within vfio while holding this spinlock, with the reference held
until the last close for the device in question.

Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
Reported-by: Anthony Krowiak <akrowiak@xxxxxxxxxxxxx>
Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
---
 drivers/vfio/group.c     | 14 +++----
 drivers/vfio/vfio.h      |  5 ++-
 drivers/vfio/vfio_main.c | 88 ++++++++++++++++++++++++++++++++++++----
 include/linux/vfio.h     |  2 +-
 4 files changed, 88 insertions(+), 21 deletions(-)

diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f0271e..8f67d33e0e8d 100644
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -164,13 +164,7 @@ static int vfio_device_group_open(struct vfio_device *device)
 		goto out_unlock;
 	}
 
-	/*
-	 * Here we pass the KVM pointer with the group under the lock.  If the
-	 * device driver will use it, it must obtain a reference and release it
-	 * during close_device.
-	 */
-	ret = vfio_device_open(device, device->group->iommufd,
-			       device->group->kvm);
+	ret = vfio_device_open(device, device->group);
 
 out_unlock:
 	mutex_unlock(&device->group->group_lock);
@@ -450,6 +444,7 @@ static struct vfio_group *vfio_group_alloc(struct iommu_group *iommu_group,
 
 	refcount_set(&group->drivers, 1);
 	mutex_init(&group->group_lock);
+	spin_lock_init(&group->kvm_ref_lock);
 	INIT_LIST_HEAD(&group->device_list);
 	mutex_init(&group->device_lock);
 	group->iommu_group = iommu_group;
@@ -799,13 +794,14 @@ EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
 void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
 {
 	struct vfio_group *group = file->private_data;
+	unsigned long flags;
 
 	if (!vfio_file_is_group(file))
 		return;
 
-	mutex_lock(&group->group_lock);
+	spin_lock_irqsave(&group->kvm_ref_lock, flags);
 	group->kvm = kvm;
-	mutex_unlock(&group->group_lock);
+	spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
 }
 EXPORT_SYMBOL_GPL(vfio_file_set_kvm);
 
diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
index f8219a438bfb..d153ad35de24 100644
--- a/drivers/vfio/vfio.h
+++ b/drivers/vfio/vfio.h
@@ -15,11 +15,11 @@ struct iommufd_ctx;
 struct iommu_group;
 struct vfio_device;
 struct vfio_container;
+struct vfio_group;
 
 void vfio_device_put_registration(struct vfio_device *device);
 bool vfio_device_try_get_registration(struct vfio_device *device);
-int vfio_device_open(struct vfio_device *device,
-		     struct iommufd_ctx *iommufd, struct kvm *kvm);
+int vfio_device_open(struct vfio_device *device, struct vfio_group *group);
 void vfio_device_close(struct vfio_device *device,
 		       struct iommufd_ctx *iommufd);
 
@@ -74,6 +74,7 @@ struct vfio_group {
 	struct file			*opened_file;
 	struct blocking_notifier_head	notifier;
 	struct iommufd_ctx		*iommufd;
+	spinlock_t			kvm_ref_lock;
 };
 
 int vfio_device_set_group(struct vfio_device *device,
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb061b17..b0defb0d0d87 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -16,6 +16,9 @@
 #include <linux/fs.h>
 #include <linux/idr.h>
 #include <linux/iommu.h>
+#ifdef CONFIG_HAVE_KVM
+#include <linux/kvm_host.h>
+#endif
 #include <linux/list.h>
 #include <linux/miscdevice.h>
 #include <linux/module.h>
@@ -344,9 +347,59 @@ static bool vfio_assert_device_open(struct vfio_device *device)
 	return !WARN_ON_ONCE(!READ_ONCE(device->open_count));
 }
 
+#ifdef CONFIG_HAVE_KVM
+static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
+{
+	void (*pfn)(struct kvm *kvm);
+	bool (*fn)(struct kvm *kvm);
+	bool ret;
+
+	pfn = symbol_get(kvm_put_kvm);
+	if (WARN_ON(!pfn))
+		return false;
+
+	fn = symbol_get(kvm_get_kvm_safe);
+	if (WARN_ON(!fn)) {
+		symbol_put(kvm_put_kvm);
+		return false;
+	}
+
+	ret = fn(kvm);
+	if (ret)
+		device->put_kvm = pfn;
+	else
+		symbol_put(kvm_put_kvm);
+
+	symbol_put(kvm_get_kvm_safe);
+
+	return ret;
+}
+
+static void vfio_kvm_put_kvm(struct vfio_device *device)
+{
+	if (WARN_ON(!device->kvm || !device->put_kvm))
+		return;
+
+	device->put_kvm(device->kvm);
+	symbol_put(kvm_put_kvm);
+}
+
+#else
+static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
+{
+	return false;
+}
+
+static void vfio_kvm_put_kvm(struct vfio_device *device)
+{
+}
+#endif
+
 static int vfio_device_first_open(struct vfio_device *device,
-				  struct iommufd_ctx *iommufd, struct kvm *kvm)
+				  struct vfio_group *group)
 {
+	struct iommufd_ctx *iommufd = group->iommufd;
+	unsigned long flags;
 	int ret;
 
 	lockdep_assert_held(&device->dev_set->lock);
@@ -361,16 +414,30 @@ static int vfio_device_first_open(struct vfio_device *device,
 	if (ret)
 		goto err_module_put;
 
-	device->kvm = kvm;
+	/*
+	 * Get the KVM pointer currently associated with the group, if there
+	 * is one, and obtain a reference now that will be held until the
+	 * open_count reaches 0.  Save the pointer in the device for use by
+	 * drivers.
+	 */
+	spin_lock_irqsave(&group->kvm_ref_lock, flags);
+	if (group->kvm && vfio_kvm_get_kvm_safe(device, group->kvm))
+		device->kvm = group->kvm;
+	spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
+
 	if (device->ops->open_device) {
 		ret = device->ops->open_device(device);
 		if (ret)
-			goto err_unuse_iommu;
+			goto err_put_kvm;
 	}
 	return 0;
 
-err_unuse_iommu:
-	device->kvm = NULL;
+err_put_kvm:
+	if (device->kvm) {
+		vfio_kvm_put_kvm(device);
+		device->put_kvm = NULL;
+		device->kvm = NULL;
+	}
 	if (iommufd)
 		vfio_iommufd_unbind(device);
 	else
@@ -387,7 +454,11 @@ static void vfio_device_last_close(struct vfio_device *device,
 
 	if (device->ops->close_device)
 		device->ops->close_device(device);
-	device->kvm = NULL;
+	if (device->kvm) {
+		vfio_kvm_put_kvm(device);
+		device->kvm = NULL;
+		device->put_kvm = NULL;
+	}
 	if (iommufd)
 		vfio_iommufd_unbind(device);
 	else
@@ -395,15 +466,14 @@ static void vfio_device_last_close(struct vfio_device *device,
 	module_put(device->dev->driver->owner);
 }
 
-int vfio_device_open(struct vfio_device *device,
-		     struct iommufd_ctx *iommufd, struct kvm *kvm)
+int vfio_device_open(struct vfio_device *device, struct vfio_group *group)
 {
 	int ret = 0;
 
 	mutex_lock(&device->dev_set->lock);
 	device->open_count++;
 	if (device->open_count == 1) {
-		ret = vfio_device_first_open(device, iommufd, kvm);
+		ret = vfio_device_first_open(device, group);
 		if (ret)
 			device->open_count--;
 	}
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index 35be78e9ae57..87ff862ff555 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -46,7 +46,6 @@ struct vfio_device {
 	struct vfio_device_set *dev_set;
 	struct list_head dev_set_list;
 	unsigned int migration_flags;
-	/* Driver must reference the kvm during open_device or never touch it */
 	struct kvm *kvm;
 
 	/* Members below here are private, not for driver use */
@@ -58,6 +57,7 @@ struct vfio_device {
 	struct list_head group_next;
 	struct list_head iommu_entry;
 	struct iommufd_access *iommufd_access;
+	void (*put_kvm)(struct kvm *kvm);
 #if IS_ENABLED(CONFIG_IOMMUFD)
 	struct iommufd_device *iommufd_device;
 	struct iommufd_ctx *iommufd_ictx;
-- 
2.39.1