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

[Jia-Ju Bai]

Thanks for the reply, Sean :)

On 2020/5/7 16:43, Sean Young wrote:
> On Tue, May 05, 2020 at 10:21:10PM +0800, Jia-Ju Bai wrote:
> > 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)) {
> Here buffer[4] is signed char, so if buffer[4] == 0 then (buffer[4] - 1) = -1,
> this becomes "if (-1 < ARRAY_SIZE(rc_keys))", which evaluates to false,
> due to it becoming an unsigned compare. _I think_.

I think you are right.
Maybe I should use "int index = buffer[4]" here.

> > -			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);
> Like Greg said, this patch reduces the number of dereferences and makes
> the code much cleaner, but the commit message is misleading.

Okay, I will change my log and send a new patch.


Best wishes,
Jia-Ju Bai