[PATCH v2] vfio: fix potential deadlock on vfio group lock

[Matthew Rosato]

Currently it is possible that the final put of a KVM reference comes from
vfio during its device close operation.  This occurs while the vfio group
lock is held; however, if the vfio device is still in the kvm device list,
then the following call chain could result in a deadlock:

kvm_put_kvm
 -> kvm_destroy_vm
  -> kvm_destroy_devices
   -> kvm_vfio_destroy
    -> kvm_vfio_file_set_kvm
     -> vfio_file_set_kvm
      -> group->group_lock/group_rwsem

Avoid this scenario by having vfio core code acquire a KVM reference
the first time a device is opened and hold that reference until the
device fd is closed, at a point after the group lock has been released.

Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
Reported-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
---
Changes from v1:
* Re-write using symbol get logic to get kvm ref during first device
  open, release the ref during device fd close after group lock is
  released
* Drop kvm get/put changes to drivers; now that vfio core holds a
  kvm ref until sometime after the device_close op is called, it
  should be fine for drivers to get and put their own references to it.
---
 drivers/vfio/group.c     |  6 ++---
 drivers/vfio/vfio_main.c | 48 +++++++++++++++++++++++++++++++++++++---
 include/linux/vfio.h     |  1 -
 3 files changed, 48 insertions(+), 7 deletions(-)

diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f0271e..2b0da82f82f4 100644
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -165,9 +165,9 @@ static int vfio_device_group_open(struct vfio_device *device)
 	}
 
 	/*
-	 * 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.
+	 * Here we pass the KVM pointer with the group under the lock.  A
+	 * reference will be obtained the first time the device is opened and
+	 * will be held until the device fd is closed.
 	 */
 	ret = vfio_device_open(device, device->group->iommufd,
 			       device->group->kvm);
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb061b17..c969e2a0ecd3 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -16,6 +16,7 @@
 #include <linux/fs.h>
 #include <linux/idr.h>
 #include <linux/iommu.h>
+#include <linux/kvm_host.h>
 #include <linux/list.h>
 #include <linux/miscdevice.h>
 #include <linux/module.h>
@@ -344,6 +345,35 @@ static bool vfio_assert_device_open(struct vfio_device *device)
 	return !WARN_ON_ONCE(!READ_ONCE(device->open_count));
 }
 
+static bool vfio_kvm_get_kvm_safe(struct kvm *kvm)
+{
+	bool (*fn)(struct kvm *kvm);
+	bool ret;
+
+	fn = symbol_get(kvm_get_kvm_safe);
+	if (WARN_ON(!fn))
+		return false;
+
+	ret = fn(kvm);
+
+	symbol_put(kvm_get_kvm_safe);
+
+	return ret;
+}
+
+static void vfio_kvm_put_kvm(struct kvm *kvm)
+{
+	void (*fn)(struct kvm *kvm);
+
+	fn = symbol_get(kvm_put_kvm);
+	if (WARN_ON(!fn))
+		return;
+
+	fn(kvm);
+
+	symbol_put(kvm_put_kvm);
+}
+
 static int vfio_device_first_open(struct vfio_device *device,
 				  struct iommufd_ctx *iommufd, struct kvm *kvm)
 {
@@ -361,16 +391,24 @@ static int vfio_device_first_open(struct vfio_device *device,
 	if (ret)
 		goto err_module_put;
 
+	if (kvm && !vfio_kvm_get_kvm_safe(kvm)) {
+		ret = -ENOENT;
+		goto err_unuse_iommu;
+	}
 	device->kvm = kvm;
 	if (device->ops->open_device) {
 		ret = device->ops->open_device(device);
 		if (ret)
-			goto err_unuse_iommu;
+			goto err_put_kvm;
 	}
 	return 0;
 
+err_put_kvm:
+	if (kvm) {
+		vfio_kvm_put_kvm(kvm);
+		device->kvm = NULL;
+	}
 err_unuse_iommu:
-	device->kvm = NULL;
 	if (iommufd)
 		vfio_iommufd_unbind(device);
 	else
@@ -387,7 +425,6 @@ 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
@@ -465,6 +502,11 @@ static int vfio_device_fops_release(struct inode *inode, struct file *filep)
 
 	vfio_device_group_close(device);
 
+	if (device->open_count == 0 && device->kvm) {
+		vfio_kvm_put_kvm(device->kvm);
+		device->kvm = NULL;
+	}
+
 	vfio_device_put_registration(device);
 
 	return 0;
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index 35be78e9ae57..3ff7e9302cc1 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 */
-- 
2.39.0