[PATCH 4.9 053/159] fbdev: smscufx: Fix use-after-free in ufx_ops_open()

[Greg Kroah-Hartman]

From: Hyunwoo Kim <imv4bel@xxxxxxxxx>

commit 5610bcfe8693c02e2e4c8b31427f1bdbdecc839c upstream.

A race condition may occur if the user physically removes the
USB device while calling open() for this device node.

This is a race condition between the ufx_ops_open() function and
the ufx_usb_disconnect() function, which may eventually result in UAF.

So, add a mutex to the ufx_ops_open() and ufx_usb_disconnect() functions
to avoid race contidion of krefs.

Signed-off-by: Hyunwoo Kim <imv4bel@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Helge Deller <deller@xxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/video/fbdev/smscufx.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

--- a/drivers/video/fbdev/smscufx.c
+++ b/drivers/video/fbdev/smscufx.c
@@ -140,6 +140,8 @@ static int ufx_submit_urb(struct ufx_dat
 static int ufx_alloc_urb_list(struct ufx_data *dev, int count, size_t size);
 static void ufx_free_urb_list(struct ufx_data *dev);
 
+static DEFINE_MUTEX(disconnect_mutex);
+
 /* reads a control register */
 static int ufx_reg_read(struct ufx_data *dev, u32 index, u32 *data)
 {
@@ -1073,9 +1075,13 @@ static int ufx_ops_open(struct fb_info *
 	if (user == 0 && !console)
 		return -EBUSY;
 
+	mutex_lock(&disconnect_mutex);
+
 	/* If the USB device is gone, we don't accept new opens */
-	if (dev->virtualized)
+	if (dev->virtualized) {
+		mutex_unlock(&disconnect_mutex);
 		return -ENODEV;
+	}
 
 	dev->fb_count++;
 
@@ -1100,6 +1106,8 @@ static int ufx_ops_open(struct fb_info *
 	pr_debug("open /dev/fb%d user=%d fb_info=%p count=%d",
 		info->node, user, info, dev->fb_count);
 
+	mutex_unlock(&disconnect_mutex);
+
 	return 0;
 }
 
@@ -1761,6 +1769,8 @@ static void ufx_usb_disconnect(struct us
 {
 	struct ufx_data *dev;
 
+	mutex_lock(&disconnect_mutex);
+
 	dev = usb_get_intfdata(interface);
 
 	pr_debug("USB disconnect starting\n");
@@ -1781,6 +1791,8 @@ static void ufx_usb_disconnect(struct us
 	kref_put(&dev->kref, ufx_free);
 
 	/* consider ufx_data freed */
+
+	mutex_unlock(&disconnect_mutex);
 }
 
 static struct usb_driver ufx_driver = {