[PATCH] media: usb: ttusb-dec: avoid buffer overflow in ttusb_dec_handle_irq() when DMA failures/attacks occur

[Jia-Ju Bai]

In ttusb_dec_init_usb():
  dec->irq_buffer = usb_alloc_coherent(...)

Thus, "dec->irq_buffer" is a DMA value, and it is assigned to "buffer"
in ttusb_dec_handle_irq():
  char *buffer = dec->irq_buffer;

When DMA failures or attacks occur, the value of buffer[4] can be
changed at any time. In this case, "buffer[4] - 1 < ARRAY_SIZE(rc_keys)"
can be first satisfied, and then the value of buffer[4] can be changed
to a large number, causing a buffer-overflow vulnerability.

To avoid the risk of this vulnerability, buffer[4] is assigned to a
non-DMA local variable "index" at the beginning of
ttusb_dec_handle_irq(), and then this variable replaces each use of
buffer[4] in the function.

Signed-off-by: Jia-Ju Bai <baijiaju1990@xxxxxxxxx>
---
 drivers/media/usb/ttusb-dec/ttusb_dec.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/media/usb/ttusb-dec/ttusb_dec.c b/drivers/media/usb/ttusb-dec/ttusb_dec.c
index 3198f9624b7c..8543c552515b 100644
--- a/drivers/media/usb/ttusb-dec/ttusb_dec.c
+++ b/drivers/media/usb/ttusb-dec/ttusb_dec.c
@@ -250,6 +250,7 @@ static void ttusb_dec_handle_irq( struct urb *urb)
 	struct ttusb_dec *dec = urb->context;
 	char *buffer = dec->irq_buffer;
 	int retval;
+	u8 index = buffer[4];
 
 	switch(urb->status) {
 		case 0: /*success*/
@@ -281,11 +282,11 @@ static void ttusb_dec_handle_irq( struct urb *urb)
 		 * this should/could be added later ...
 		 * for now lets report each signal as a key down and up
 		 */
-		if (buffer[4] - 1 < ARRAY_SIZE(rc_keys)) {
-			dprintk("%s:rc signal:%d\n", __func__, buffer[4]);
-			input_report_key(dec->rc_input_dev, rc_keys[buffer[4] - 1], 1);
+		if (index - 1 < ARRAY_SIZE(rc_keys)) {
+			dprintk("%s:rc signal:%d\n", __func__, index);
+			input_report_key(dec->rc_input_dev, rc_keys[index - 1], 1);
 			input_sync(dec->rc_input_dev);
-			input_report_key(dec->rc_input_dev, rc_keys[buffer[4] - 1], 0);
+			input_report_key(dec->rc_input_dev, rc_keys[index - 1], 0);
 			input_sync(dec->rc_input_dev);
 		}
 	}
-- 
2.17.1