[PATCH v3] vduse: hold vduse_lock across IDR lookup in open path

[Qihang Tang <mst@redhat.com>]

vduse_dev_open() looks up struct vduse_dev through the IDR and then
acquires dev->lock only after vduse_lock has been dropped.

This leaves a window where a concurrent VDUSE_DESTROY_DEV can remove the
same object from the IDR and free it before the open path locks the
device, leading to a use-after-free.

Close this race by keeping vduse_lock held until dev->lock has been
acquired in the open path, matching the lock ordering already used by
the destroy path.

Fixes: c8a6153b6c59 ("vduse: Introduce VDUSE - vDPA Device in Userspace")
Signed-off-by: Qihang Tang <q.h.hack.winter@gmail.com>
---
v2 -> v3:
- keep vduse_lock held until after dropping dev->lock
in vduse_dev_open()
- add changelog requested in review

v1 -> v2:
- add Fixes tag
- remove helper and inline the locking in
vduse_dev_open()

 drivers/vdpa/vdpa_user/vduse_dev.c | 21 +++++++--------------
 1 file changed, 7 insertions(+), 14 deletions(-)

diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
index 6202f6902fcd..d5c34260ed68 100644
--- a/drivers/vdpa/vdpa_user/vduse_dev.c
+++ b/drivers/vdpa/vdpa_user/vduse_dev.c
@@ -1637,26 +1637,18 @@ static int vduse_dev_release(struct inode *inode, struct file *file)
 	return 0;
 }
 
-static struct vduse_dev *vduse_dev_get_from_minor(int minor)
+static int vduse_dev_open(struct inode *inode, struct file *file)
 {
+	int ret = -EBUSY;
 	struct vduse_dev *dev;
 
 	mutex_lock(&vduse_lock);
-	dev = idr_find(&vduse_idr, minor);
-	mutex_unlock(&vduse_lock);
-
-	return dev;
-}
-
-static int vduse_dev_open(struct inode *inode, struct file *file)
-{
-	int ret;
-	struct vduse_dev *dev = vduse_dev_get_from_minor(iminor(inode));
-
-	if (!dev)
+	dev = idr_find(&vduse_idr, iminor(inode));
+	if (!dev) {
+		mutex_unlock(&vduse_lock);
 		return -ENODEV;
+	}
 
-	ret = -EBUSY;
 	mutex_lock(&dev->lock);
 	if (dev->connected)
 		goto unlock;
@@ -1666,6 +1658,7 @@ static int vduse_dev_open(struct inode *inode, struct file *file)
 	file->private_data = dev;
 unlock:
 	mutex_unlock(&dev->lock);
+	mutex_unlock(&vduse_lock);
 
 	return ret;
 }
-- 
2.39.5 (Apple Git-154)