[PATCH 3.16 036/204] USB: gadgetfs: fix copy_to_user while holding spinlock

[Ben Hutchings]

3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

commit 6e76c01e71551cb221c1f3deacb9dcd9a7346784 upstream.

The gadgetfs driver as a long-outstanding FIXME, regarding a call of
copy_to_user() made while holding a spinlock.  This patch fixes the
issue by dropping the spinlock and using the dev->udc_usage mechanism
introduced by another recent patch to guard against status changes
while the lock isn't held.

Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Reported-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Acked-by: Felipe Balbi <felipe.balbi@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/usb/gadget/inode.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/drivers/usb/gadget/inode.c
+++ b/drivers/usb/gadget/inode.c
@@ -1055,11 +1055,14 @@ ep0_read (struct file *fd, char __user *
 				retval = -EIO;
 			else {
 				len = min (len, (size_t)dev->req->actual);
-// FIXME don't call this with the spinlock held ...
+				++dev->udc_usage;
+				spin_unlock_irq(&dev->lock);
 				if (copy_to_user (buf, dev->req->buf, len))
 					retval = -EFAULT;
 				else
 					retval = len;
+				spin_lock_irq(&dev->lock);
+				--dev->udc_usage;
 				clean_req (dev->gadget->ep0, dev->req);
 				/* NOTE userspace can't yet choose to stall */
 			}