Re: [PATCH v4 1/2] vfio: fix deadlock between group lock and kvm lock

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Tested-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>

On 2/3/23 4:50 PM, Matthew Rosato wrote:
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     | 44 +++++++++++++++++++++++-----
  drivers/vfio/vfio.h      | 15 ++++++++++
  drivers/vfio/vfio_main.c | 63 +++++++++++++++++++++++++++++++++++-----
  include/linux/vfio.h     |  2 +-
  4 files changed, 109 insertions(+), 15 deletions(-)

diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f0271e..98621ac082f0 100644
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -154,6 +154,18 @@ static int vfio_group_ioctl_set_container(struct vfio_group *group,
  	return ret;
  }
+static void vfio_device_group_get_kvm_safe(struct vfio_device *device)
+{
+	spin_lock(&device->group->kvm_ref_lock);
+	if (!device->group->kvm)
+		goto unlock;
+
+	_vfio_device_get_kvm_safe(device, device->group->kvm);
+
+unlock:
+	spin_unlock(&device->group->kvm_ref_lock);
+}
+
  static int vfio_device_group_open(struct vfio_device *device)
  {
  	int ret;
@@ -164,13 +176,23 @@ static int vfio_device_group_open(struct vfio_device *device)
  		goto out_unlock;
  	}
+ mutex_lock(&device->dev_set->lock);
+
  	/*
-	 * 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.
+	 * Before the first device open, 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 again.  Save
+	 * the pointer in the device for use by drivers.
  	 */
-	ret = vfio_device_open(device, device->group->iommufd,
-			       device->group->kvm);
+	if (device->open_count == 0)
+		vfio_device_group_get_kvm_safe(device);
+
+	ret = vfio_device_open(device, device->group->iommufd, device->kvm);
+
+	if (device->open_count == 0)
+		vfio_device_put_kvm(device);
+
+	mutex_unlock(&device->dev_set->lock);
out_unlock:
  	mutex_unlock(&device->group->group_lock);
@@ -180,7 +202,14 @@ static int vfio_device_group_open(struct vfio_device *device)
  void vfio_device_group_close(struct vfio_device *device)
  {
  	mutex_lock(&device->group->group_lock);
+	mutex_lock(&device->dev_set->lock);
+
  	vfio_device_close(device, device->group->iommufd);
+
+	if (device->open_count == 0)
+		vfio_device_put_kvm(device);
+
+	mutex_unlock(&device->dev_set->lock);
  	mutex_unlock(&device->group->group_lock);
  }
@@ -450,6 +479,7 @@ 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;
@@ -803,9 +833,9 @@ void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
  	if (!vfio_file_is_group(file))
  		return;
- mutex_lock(&group->group_lock);
+	spin_lock(&group->kvm_ref_lock);
  	group->kvm = kvm;
-	mutex_unlock(&group->group_lock);
+	spin_unlock(&group->kvm_ref_lock);
  }
  EXPORT_SYMBOL_GPL(vfio_file_set_kvm);
diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
index f8219a438bfb..24d6cd285945 100644
--- a/drivers/vfio/vfio.h
+++ b/drivers/vfio/vfio.h
@@ -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,
@@ -251,4 +252,18 @@ extern bool vfio_noiommu __read_mostly;
  enum { vfio_noiommu = false };
  #endif
+#ifdef CONFIG_HAVE_KVM
+void _vfio_device_get_kvm_safe(struct vfio_device *device, struct kvm *kvm);
+void vfio_device_put_kvm(struct vfio_device *device);
+#else
+static inline void _vfio_device_get_kvm_safe(struct vfio_device *device,
+					     struct kvm *kvm)
+{
+}
+
+static inline void vfio_device_put_kvm(struct vfio_device *device)
+{
+}
+#endif
+
  #endif
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb061b17..28c47cd6a6b5 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>
@@ -338,6 +341,55 @@ void vfio_unregister_group_dev(struct vfio_device *device)
  }
  EXPORT_SYMBOL_GPL(vfio_unregister_group_dev);
+#ifdef CONFIG_HAVE_KVM
+void _vfio_device_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
+{
+	void (*pfn)(struct kvm *kvm);
+	bool (*fn)(struct kvm *kvm);
+	bool ret;
+
+	lockdep_assert_held(&device->dev_set->lock);
+
+	pfn = symbol_get(kvm_put_kvm);
+	if (WARN_ON(!pfn))
+		return;
+
+	fn = symbol_get(kvm_get_kvm_safe);
+	if (WARN_ON(!fn)) {
+		symbol_put(kvm_put_kvm);
+		return;
+	}
+
+	ret = fn(kvm);
+	symbol_put(kvm_get_kvm_safe);
+	if (!ret) {
+		symbol_put(kvm_put_kvm);
+		return;
+	}
+
+	device->put_kvm = pfn;
+	device->kvm = kvm;
+}
+
+void vfio_device_put_kvm(struct vfio_device *device)
+{
+	lockdep_assert_held(&device->dev_set->lock);
+
+	if (!device->kvm)
+		return;
+
+	if (WARN_ON(!device->put_kvm))
+		goto clear;
+
+	device->put_kvm(device->kvm);
+	device->put_kvm = NULL;
+	symbol_put(kvm_put_kvm);
+
+clear:
+	device->kvm = NULL;
+}
+#endif
+
  /* true if the vfio_device has open_device() called but not close_device() */
  static bool vfio_assert_device_open(struct vfio_device *device)
  {
@@ -361,7 +413,6 @@ static int vfio_device_first_open(struct vfio_device *device,
  	if (ret)
  		goto err_module_put;
- device->kvm = kvm;
  	if (device->ops->open_device) {
  		ret = device->ops->open_device(device);
  		if (ret)
@@ -370,7 +421,6 @@ static int vfio_device_first_open(struct vfio_device *device,
  	return 0;
err_unuse_iommu:
-	device->kvm = NULL;
  	if (iommufd)
  		vfio_iommufd_unbind(device);
  	else
@@ -387,7 +437,6 @@ static void vfio_device_last_close(struct vfio_device *device,
if (device->ops->close_device)
  		device->ops->close_device(device);
-	device->kvm = NULL;
  	if (iommufd)
  		vfio_iommufd_unbind(device);
  	else
@@ -400,14 +449,14 @@ int vfio_device_open(struct vfio_device *device,
  {
  	int ret = 0;
- mutex_lock(&device->dev_set->lock);
+	lockdep_assert_held(&device->dev_set->lock);
+
  	device->open_count++;
  	if (device->open_count == 1) {
  		ret = vfio_device_first_open(device, iommufd, kvm);
  		if (ret)
  			device->open_count--;
  	}
-	mutex_unlock(&device->dev_set->lock);
return ret;
  }
@@ -415,12 +464,12 @@ int vfio_device_open(struct vfio_device *device,
  void vfio_device_close(struct vfio_device *device,
  		       struct iommufd_ctx *iommufd)
  {
-	mutex_lock(&device->dev_set->lock);
+	lockdep_assert_held(&device->dev_set->lock);
+
  	vfio_assert_device_open(device);
  	if (device->open_count == 1)
  		vfio_device_last_close(device, iommufd);
  	device->open_count--;
-	mutex_unlock(&device->dev_set->lock);
  }
/*
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;



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