media: pci: tw68: Fix null-ptr-deref bug in buf prepare and finish

When the driver calls tw68_risc_buffer() to prepare the buffer, the
function call dma_alloc_coherent may fail, resulting in a empty buffer
buf->cpu. Later when we free the buffer or access the buffer, null ptr
deref is triggered.

This bug is similar to the following one:
https://git.linuxtv.org/media_stage.git/commit/?id=2b064d91440b33fba5b452f2d1b31f13ae911d71.

We believe the bug can be also dynamically triggered from user side.
Similarly, we fix this by checking the return value of tw68_risc_buffer()
and the value of buf->cpu before buffer free.
Signed-off-by: harperchen <harperchen1110@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/pci/tw68/tw68-video.c

@@ -437,6 +437,7 @@ static void tw68_buf_queue(struct vb2_buffer *vb)
  */
 static int tw68_buf_prepare(struct vb2_buffer *vb)
 {
+	int ret;
 	struct vb2_v4l2_buffer *vbuf = to_vb2_v4l2_buffer(vb);
 	struct vb2_queue *vq = vb->vb2_queue;
 	struct tw68_dev *dev = vb2_get_drv_priv(vq);
@@ -452,30 +453,30 @@ static int tw68_buf_prepare(struct vb2_buffer *vb)
 	bpl = (dev->width * dev->fmt->depth) >> 3;
 	switch (dev->field) {
 	case V4L2_FIELD_TOP:
-		tw68_risc_buffer(dev->pci, buf, dma->sgl,
+		ret = tw68_risc_buffer(dev->pci, buf, dma->sgl,
 				 0, UNSET, bpl, 0, dev->height);
 		break;
 	case V4L2_FIELD_BOTTOM:
-		tw68_risc_buffer(dev->pci, buf, dma->sgl,
+		ret = tw68_risc_buffer(dev->pci, buf, dma->sgl,
 				 UNSET, 0, bpl, 0, dev->height);
 		break;
 	case V4L2_FIELD_SEQ_TB:
-		tw68_risc_buffer(dev->pci, buf, dma->sgl,
+		ret = tw68_risc_buffer(dev->pci, buf, dma->sgl,
 				 0, bpl * (dev->height >> 1),
 				 bpl, 0, dev->height >> 1);
 		break;
 	case V4L2_FIELD_SEQ_BT:
-		tw68_risc_buffer(dev->pci, buf, dma->sgl,
+		ret = tw68_risc_buffer(dev->pci, buf, dma->sgl,
 				 bpl * (dev->height >> 1), 0,
 				 bpl, 0, dev->height >> 1);
 		break;
 	case V4L2_FIELD_INTERLACED:
 	default:
-		tw68_risc_buffer(dev->pci, buf, dma->sgl,
+		ret = tw68_risc_buffer(dev->pci, buf, dma->sgl,
 				 0, bpl, bpl, bpl, dev->height >> 1);
 		break;
 	}
-	return 0;
+	return ret;
 }
 
 static void tw68_buf_finish(struct vb2_buffer *vb)
@@ -485,6 +486,7 @@ static void tw68_buf_finish(struct vb2_buffer *vb)
 	struct tw68_dev *dev = vb2_get_drv_priv(vq);
 	struct tw68_buf *buf = container_of(vbuf, struct tw68_buf, vb);
 
+	if (buf->cpu)
 		dma_free_coherent(&dev->pci->dev, buf->size, buf->cpu, buf->dma);
 }