dvb_frontend_open() calls dvb_generic_release() in its error path after
dvb_generic_open() succeeds. dvb_generic_release() drops the device
reference via dvb_device_put(), and then dvb_device_open() drops it again
in its error handling, causing a use-after-free and refcount underflow.

Fix this by introducing __dvb_generic_release() which only restores the
users/readers/writers counters without dropping the device reference. Use
it in dvb_frontend_open()'s error path so that dvb_device_open() remains
the sole owner of the dvb_device_put() on open failure.

Reported-by: syzbot+40339ea82afa8184ad5d@syzkaller.appspotmail.com
Fixes: 0fc044b2b5e2 ("media: dvb-core: Fix use-after-free on race condition at dvb_frontend")
Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
---
 drivers/media/dvb-core/dvb_frontend.c |  2 +-
 drivers/media/dvb-core/dvbdev.c       | 17 ++++++++++++-----
 include/media/dvbdev.h                | 12 ++++++++++++
 3 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/media/dvb-core/dvb_frontend.c b/drivers/media/dvb-core/dvb_frontend.c
index d082b6c57c76..497f5920b267 100644
--- a/drivers/media/dvb-core/dvb_frontend.c
+++ b/drivers/media/dvb-core/dvb_frontend.c
@@ -2887,7 +2887,7 @@ static int dvb_frontend_open(struct inode *inode, struct file *file)
 	mutex_unlock(&fe->dvb->mdev_lock);
 err2:
 #endif
-	dvb_generic_release(inode, file);
+	__dvb_generic_release(inode, file);
 err1:
 	if (dvbdev->users == -1 && fe->ops.ts_bus_ctrl)
 		fe->ops.ts_bus_ctrl(fe, 0);
diff --git a/drivers/media/dvb-core/dvbdev.c b/drivers/media/dvb-core/dvbdev.c
index d753d329502a..3e0ad67b79a3 100644
--- a/drivers/media/dvb-core/dvbdev.c
+++ b/drivers/media/dvb-core/dvbdev.c
@@ -152,19 +152,26 @@ int dvb_generic_open(struct inode *inode, struct file *file)
 }
 EXPORT_SYMBOL(dvb_generic_open);
 
-int dvb_generic_release(struct inode *inode, struct file *file)
+void __dvb_generic_release(struct inode *inode, struct file *file)
 {
 	struct dvb_device *dvbdev = file->private_data;
 
-	if (!dvbdev)
-		return -ENODEV;
-
 	if ((file->f_flags & O_ACCMODE) == O_RDONLY)
 		dvbdev->readers++;
 	else
 		dvbdev->writers++;
-
 	dvbdev->users++;
+}
+EXPORT_SYMBOL(__dvb_generic_release);
+
+int dvb_generic_release(struct inode *inode, struct file *file)
+{
+	struct dvb_device *dvbdev = file->private_data;
+
+	if (!dvbdev)
+		return -ENODEV;
+
+	__dvb_generic_release(inode, file);
 
 	dvb_device_put(dvbdev);
 
diff --git a/include/media/dvbdev.h b/include/media/dvbdev.h
index e5a00d126612..9e6e5cb43dcb 100644
--- a/include/media/dvbdev.h
+++ b/include/media/dvbdev.h
@@ -343,6 +343,18 @@ int dvb_create_media_graph(struct dvb_adapter *adap,
  */
 int dvb_generic_open(struct inode *inode, struct file *file);
 
+/*
+ * __dvb_generic_release - Undo dvb_generic_open() counters WITHOUT
+ *      dropping the device reference.
+ *
+ * @inode: pointer to &struct inode.
+ * @file: pointer to &struct file.
+ *
+ * Used in cases where the caller handles dvb_device_put() and ensures
+ * that dvbdev is valid.
+ */
+void __dvb_generic_release(struct inode *inode, struct file *file);
+
 /**
  * dvb_generic_release - Digital TV close function, used by DVB devices
  *
-- 
2.43.0