[PATCH] USB: chaoskey: fail open after removal

[Oliver Neukum <oneukum@suse.com>]

chaoskey_open() takes the lock only to increase the
counter of openings. That means that the mutual exclusion
with chaoskey_disconnect() cannot prevent an increase
of the counter and chaoskey_open() returning a success.

If that race is hit, chaoskey_disconnect() will happily
free all resources associated with the device after
it has dropped the lock, as it has read the counter
as zero.

To prevent this race chaoskey_open() has to check
the presence of the device under the lock.
However, the current per device lock cannot be used,
because it is a part of the data structure to be
freed. Hence an additional global mutex is needed.
The issue is as old as the driver.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Reported-by: syzbot+422188bce66e76020e55@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=422188bce66e76020e55
Fixes: 66e3e591891da ("usb: Add driver for Altus Metrum ChaosKey device (v2)")
---
 drivers/usb/misc/chaoskey.c | 35 ++++++++++++++++++++++++-----------
 1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/drivers/usb/misc/chaoskey.c b/drivers/usb/misc/chaoskey.c
index 6fb5140e29b9..e8b63df5f975 100644
--- a/drivers/usb/misc/chaoskey.c
+++ b/drivers/usb/misc/chaoskey.c
@@ -27,6 +27,8 @@ static struct usb_class_driver chaoskey_class;
 static int chaoskey_rng_read(struct hwrng *rng, void *data,
 			     size_t max, bool wait);
 
+static DEFINE_MUTEX(chaoskey_list_lock);
+
 #define usb_dbg(usb_if, format, arg...) \
 	dev_dbg(&(usb_if)->dev, format, ## arg)
 
@@ -230,6 +232,7 @@ static void chaoskey_disconnect(struct usb_interface *interface)
 	if (dev->hwrng_registered)
 		hwrng_unregister(&dev->hwrng);
 
+	mutex_lock(&chaoskey_list_lock);
 	usb_deregister_dev(interface, &chaoskey_class);
 
 	usb_set_intfdata(interface, NULL);
@@ -244,6 +247,7 @@ static void chaoskey_disconnect(struct usb_interface *interface)
 	} else
 		mutex_unlock(&dev->lock);
 
+	mutex_unlock(&chaoskey_list_lock);
 	usb_dbg(interface, "disconnect done");
 }
 
@@ -251,6 +255,7 @@ static int chaoskey_open(struct inode *inode, struct file *file)
 {
 	struct chaoskey *dev;
 	struct usb_interface *interface;
+	int rv = 0;
 
 	/* get the interface from minor number and driver information */
 	interface = usb_find_interface(&chaoskey_driver, iminor(inode));
@@ -266,18 +271,23 @@ static int chaoskey_open(struct inode *inode, struct file *file)
 	}
 
 	file->private_data = dev;
+	mutex_lock(&chaoskey_list_lock);
 	mutex_lock(&dev->lock);
-	++dev->open;
+	if (dev->present)
+		++dev->open;
+	else
+		rv = -ENODEV;
 	mutex_unlock(&dev->lock);
+	mutex_unlock(&chaoskey_list_lock);
 
-	usb_dbg(interface, "open success");
-	return 0;
+	return rv;
 }
 
 static int chaoskey_release(struct inode *inode, struct file *file)
 {
 	struct chaoskey *dev = file->private_data;
 	struct usb_interface *interface;
+	int rv = 0;
 
 	if (dev == NULL)
 		return -ENODEV;
@@ -286,14 +296,15 @@ static int chaoskey_release(struct inode *inode, struct file *file)
 
 	usb_dbg(interface, "release");
 
+	mutex_lock(&chaoskey_list_lock);
 	mutex_lock(&dev->lock);
 
 	usb_dbg(interface, "open count at release is %d", dev->open);
 
 	if (dev->open <= 0) {
 		usb_dbg(interface, "invalid open count (%d)", dev->open);
-		mutex_unlock(&dev->lock);
-		return -ENODEV;
+		rv = -ENODEV;
+		goto bail;
 	}
 
 	--dev->open;
@@ -302,13 +313,15 @@ static int chaoskey_release(struct inode *inode, struct file *file)
 		if (dev->open == 0) {
 			mutex_unlock(&dev->lock);
 			chaoskey_free(dev);
-		} else
-			mutex_unlock(&dev->lock);
-	} else
-		mutex_unlock(&dev->lock);
-
+			goto destruction;
+		}
+	}
+bail:
+	mutex_unlock(&dev->lock);
+destruction:
+	mutex_lock(&chaoskey_list_lock);
 	usb_dbg(interface, "release success");
-	return 0;
+	return rv;
 }
 
 static void chaos_read_callback(struct urb *urb)
-- 
2.46.1