[PATCH 2/2] media: usbvision: Fix races among open, close, and disconnect

[Alan Stern <stern@rowland.harvard.edu>]

Visual inspection of the usbvision driver shows that it suffers from
three races between its open, close, and disconnect handlers.  In
particular, the driver is careful to update its usbvision->user and
usbvision->remove_pending flags while holding the private mutex, but:

	usbvision_v4l2_close() and usbvision_radio_close() don't hold
	the mutex while they check the value of
	usbvision->remove_pending;

	usbvision_disconnect() doesn't hold the mutex while checking
	the value of usbvision->user; and

	also, usbvision_v4l2_open() and usbvision_radio_open() don't
	check whether the device has been unplugged before allowing
	the user to open the device files.

Each of these can potentially lead to usbvision_release() being called
twice and use-after-free errors.

This patch fixes the races by reading the flags while the mutex is
still held and checking for pending removes before allowing an open to
succeed.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: <stable@vger.kernel.org>

---

[as1920]


 drivers/media/usb/usbvision/usbvision-video.c |   21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

Index: usb-devel/drivers/media/usb/usbvision/usbvision-video.c
===================================================================
--- usb-devel.orig/drivers/media/usb/usbvision/usbvision-video.c
+++ usb-devel/drivers/media/usb/usbvision/usbvision-video.c
@@ -314,6 +314,10 @@ static int usbvision_v4l2_open(struct fi
 	if (mutex_lock_interruptible(&usbvision->v4l2_lock))
 		return -ERESTARTSYS;
 
+	if (usbvision->remove_pending) {
+		err_code = -ENODEV;
+		goto unlock;
+	}
 	if (usbvision->user) {
 		err_code = -EBUSY;
 	} else {
@@ -377,6 +381,7 @@ unlock:
 static int usbvision_v4l2_close(struct file *file)
 {
 	struct usb_usbvision *usbvision = video_drvdata(file);
+	int r;
 
 	PDEBUG(DBG_IO, "close");
 
@@ -391,9 +396,10 @@ static int usbvision_v4l2_close(struct f
 	usbvision_scratch_free(usbvision);
 
 	usbvision->user--;
+	r = usbvision->remove_pending;
 	mutex_unlock(&usbvision->v4l2_lock);
 
-	if (usbvision->remove_pending) {
+	if (r) {
 		printk(KERN_INFO "%s: Final disconnect\n", __func__);
 		usbvision_release(usbvision);
 		return 0;
@@ -1076,6 +1082,11 @@ static int usbvision_radio_open(struct f
 
 	if (mutex_lock_interruptible(&usbvision->v4l2_lock))
 		return -ERESTARTSYS;
+
+	if (usbvision->remove_pending) {
+		err_code = -ENODEV;
+		goto out;
+	}
 	err_code = v4l2_fh_open(file);
 	if (err_code)
 		goto out;
@@ -1108,6 +1119,7 @@ out:
 static int usbvision_radio_close(struct file *file)
 {
 	struct usb_usbvision *usbvision = video_drvdata(file);
+	int r;
 
 	PDEBUG(DBG_IO, "");
 
@@ -1121,9 +1133,10 @@ static int usbvision_radio_close(struct
 	usbvision_audio_off(usbvision);
 	usbvision->radio = 0;
 	usbvision->user--;
+	r = usbvision->remove_pending;
 	mutex_unlock(&usbvision->v4l2_lock);
 
-	if (usbvision->remove_pending) {
+	if (r) {
 		printk(KERN_INFO "%s: Final disconnect\n", __func__);
 		v4l2_fh_release(file);
 		usbvision_release(usbvision);
@@ -1555,6 +1568,7 @@ err_usb:
 static void usbvision_disconnect(struct usb_interface *intf)
 {
 	struct usb_usbvision *usbvision = to_usbvision(usb_get_intfdata(intf));
+	int u;
 
 	PDEBUG(DBG_PROBE, "");
 
@@ -1571,13 +1585,14 @@ static void usbvision_disconnect(struct
 	v4l2_device_disconnect(&usbvision->v4l2_dev);
 	usbvision_i2c_unregister(usbvision);
 	usbvision->remove_pending = 1;	/* Now all ISO data will be ignored */
+	u = usbvision->user;
 
 	usb_put_dev(usbvision->dev);
 	usbvision->dev = NULL;	/* USB device is no more */
 
 	mutex_unlock(&usbvision->v4l2_lock);
 
-	if (usbvision->user) {
+	if (u) {
 		printk(KERN_INFO "%s: In use, disconnect pending\n",
 		       __func__);
 		wake_up_interruptible(&usbvision->wait_frame);