media: cx23885: Fix a null-ptr-deref bug in buffer_prepare() and buffer_finish()

When the driver calls cx23885_risc_buffer() to prepare the buffer, the
function call dma_alloc_coherent may fail, resulting in a empty buffer
risc->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 cx23885_risc_buffer()
and the value of risc->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/cx23885/cx23885-core.c

@@ -1325,7 +1325,9 @@ void cx23885_free_buffer(struct cx23885_dev *dev, struct cx23885_buffer *buf)
 {
 	struct cx23885_riscmem *risc = &buf->risc;
 
-	dma_free_coherent(&dev->pci->dev, risc->size, risc->cpu, risc->dma);
+	if (risc->cpu)
+		dma_free_coherent(&dev->pci->dev, risc->size, risc->cpu, risc->dma);
+	memset(risc, 0, sizeof(*risc));
 }
 
 static void cx23885_tsport_reg_dump(struct cx23885_tsport *port)

drivers/media/pci/cx23885/cx23885-video.c

@@ -342,6 +342,7 @@ static int queue_setup(struct vb2_queue *q,
 
 static int buffer_prepare(struct vb2_buffer *vb)
 {
+	int ret;
 	struct vb2_v4l2_buffer *vbuf = to_vb2_v4l2_buffer(vb);
 	struct cx23885_dev *dev = vb->vb2_queue->drv_priv;
 	struct cx23885_buffer *buf =
@@ -358,12 +359,12 @@ static int buffer_prepare(struct vb2_buffer *vb)
 
 	switch (dev->field) {
 	case V4L2_FIELD_TOP:
-		cx23885_risc_buffer(dev->pci, &buf->risc,
+		ret = cx23885_risc_buffer(dev->pci, &buf->risc,
 				sgt->sgl, 0, UNSET,
 				buf->bpl, 0, dev->height);
 		break;
 	case V4L2_FIELD_BOTTOM:
-		cx23885_risc_buffer(dev->pci, &buf->risc,
+		ret = cx23885_risc_buffer(dev->pci, &buf->risc,
 				sgt->sgl, UNSET, 0,
 				buf->bpl, 0, dev->height);
 		break;
@@ -391,21 +392,21 @@ static int buffer_prepare(struct vb2_buffer *vb)
 			line0_offset = 0;
 			line1_offset = buf->bpl;
 		}
-		cx23885_risc_buffer(dev->pci, &buf->risc,
+		ret = cx23885_risc_buffer(dev->pci, &buf->risc,
 				sgt->sgl, line0_offset,
 				line1_offset,
 				buf->bpl, buf->bpl,
 				dev->height >> 1);
 		break;
 	case V4L2_FIELD_SEQ_TB:
-		cx23885_risc_buffer(dev->pci, &buf->risc,
+		ret = cx23885_risc_buffer(dev->pci, &buf->risc,
 				sgt->sgl,
 				0, buf->bpl * (dev->height >> 1),
 				buf->bpl, 0,
 				dev->height >> 1);
 		break;
 	case V4L2_FIELD_SEQ_BT:
-		cx23885_risc_buffer(dev->pci, &buf->risc,
+		ret = cx23885_risc_buffer(dev->pci, &buf->risc,
 				sgt->sgl,
 				buf->bpl * (dev->height >> 1), 0,
 				buf->bpl, 0,
@@ -418,7 +419,7 @@ static int buffer_prepare(struct vb2_buffer *vb)
 		buf, buf->vb.vb2_buf.index,
 		dev->width, dev->height, dev->fmt->depth, dev->fmt->fourcc,
 		(unsigned long)buf->risc.dma);
-	return 0;
+	return ret;
 }
 
 static void buffer_finish(struct vb2_buffer *vb)