[PATCH v2] usb: gadget: printer: fix infinite loop in printer_read()

[Melbin K Mathew <mlbnkm1@gmail.com>]

printer_read() uses the same variable for the requested copy size and
the number of bytes actually copied to user space. copy_to_user()
returns the number of bytes not copied, so when it fails to copy
anything, the computed copied length becomes zero.

In that case len, buf, current_rx_bytes and current_rx_buf are left
unchanged. If RX data is available and the user buffer remains
unwritable, the read loop can repeat indefinitely.

Track the copied length separately and return -EFAULT, or the number of
bytes already copied, if an iteration makes no progress.

Fixes: b185f01a9ab7 ("usb: gadget: printer: factor out f_printer")
Cc: stable@vger.kernel.org
Reviewed-by: Peter Chen <peter.chen@kernel.org>
Signed-off-by: Melbin K Mathew <mlbnkm1@gmail.com>
---
Changes in v2:
- Drop unrelated comment wording change.
- Add Reviewed-by tag from Peter Chen.

 drivers/usb/gadget/function/f_printer.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/gadget/function/f_printer.c b/drivers/usb/gadget/function/f_printer.c
index e4f7828ae7..e346e4c26e 100644
--- a/drivers/usb/gadget/function/f_printer.c
+++ b/drivers/usb/gadget/function/f_printer.c
@@ -432,7 +432,7 @@ printer_read(struct file *fd, char __user *buf, size_t len, loff_t *ptr)
 {
 	struct printer_dev		*dev = fd->private_data;
 	unsigned long			flags;
-	size_t				size;
+	size_t				size, not_copied, copied;
 	size_t				bytes_copied;
 	struct usb_request		*req;
 	/* This is a pointer to the current USB rx request. */
@@ -525,14 +525,16 @@ printer_read(struct file *fd, char __user *buf, size_t len, loff_t *ptr)
 		else
 			size = len;
 
-		size -= copy_to_user(buf, current_rx_buf, size);
-		bytes_copied += size;
-		len -= size;
-		buf += size;
+		not_copied = copy_to_user(buf, current_rx_buf, size);
+		copied = size - not_copied;
+
+		bytes_copied += copied;
+		len -= copied;
+		buf += copied;
 
 		spin_lock_irqsave(&dev->lock, flags);
 
 		if (dev->reset_printer) {
 			list_add(&current_rx_req->list, &dev->rx_reqs);
 			spin_unlock_irqrestore(&dev->lock, flags);
@@ -543,6 +545,17 @@ printer_read(struct file *fd, char __user *buf, size_t len, loff_t *ptr)
 		if (dev->interface < 0)
 			goto out_disabled;
 
+		if (!copied) {
+			dev->current_rx_req = current_rx_req;
+			dev->current_rx_bytes = current_rx_bytes;
+			dev->current_rx_buf = current_rx_buf;
+			spin_unlock_irqrestore(&dev->lock, flags);
+			mutex_unlock(&dev->lock_printer_io);
+			return bytes_copied ? bytes_copied : -EFAULT;
+		}
+
+		size = copied;
+
 		/* If we not returning all the data left in this RX request
 		 * buffer then adjust the amount of data left in the buffer.
 		 * Othewise if we are done with this RX request buffer then
-- 
2.39.5