media: omap_vout: potential buffer overflow in vidioc_dqbuf()

The "b->index" is a u32 the comes from the user in the ioctl.  It hasn't
been checked.  We aren't supposed to use it but we're instead supposed
to use the value that gets written to it when we call videobuf_dqbuf().

The videobuf_dqbuf() first memsets it to zero and then re-initializes it
inside the videobuf_status() function.  It's this final value which we
want.

Hans Verkuil pointed out that we need to check the return from
videobuf_dqbuf().  I ended up doing a little cleanup related to that as
well.

Fixes: 72915e85 ("[media] V4L2: OMAP: VOUT: dma map and unmap v4l2 buffers in qbuf and dqbuf")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/platform/omap/omap_vout.c

@@ -1527,23 +1527,20 @@ static int vidioc_dqbuf(struct file *file, void *fh, struct v4l2_buffer *b)
 	unsigned long size;
 	struct videobuf_buffer *vb;
 
-	vb = q->bufs[b->index];
-
 	if (!vout->streaming)
 		return -EINVAL;
 
-	if (file->f_flags & O_NONBLOCK)
-		/* Call videobuf_dqbuf for non blocking mode */
-		ret = videobuf_dqbuf(q, (struct v4l2_buffer *)b, 1);
-	else
-		/* Call videobuf_dqbuf for  blocking mode */
-		ret = videobuf_dqbuf(q, (struct v4l2_buffer *)b, 0);
+	ret = videobuf_dqbuf(q, b, !!(file->f_flags & O_NONBLOCK));
+	if (ret)
+		return ret;
+
+	vb = q->bufs[b->index];
 
 	addr = (unsigned long) vout->buf_phy_addr[vb->i];
 	size = (unsigned long) vb->size;
 	dma_unmap_single(vout->vid_dev->v4l2_dev.dev,  addr,
 				size, DMA_TO_DEVICE);
-	return ret;
+	return 0;
 }
 
 static int vidioc_streamon(struct file *file, void *fh, enum v4l2_buf_type i)