media: cx88: Fix a null-ptr-deref bug in buffer_prepare()

When the driver calls cx88_risc_buffer() to prepare the buffer, the
function call may fail, resulting in a empty buffer and null-ptr-deref
later in buffer_queue().

The following log can reveal it:

[   41.822762] general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN PTI
[   41.824488] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[   41.828027] RIP: 0010:buffer_queue+0xc2/0x500
[   41.836311] Call Trace:
[   41.836945]  __enqueue_in_driver+0x141/0x360
[   41.837262]  vb2_start_streaming+0x62/0x4a0
[   41.838216]  vb2_core_streamon+0x1da/0x2c0
[   41.838516]  __vb2_init_fileio+0x981/0xbc0
[   41.839141]  __vb2_perform_fileio+0xbf9/0x1120
[   41.840072]  vb2_fop_read+0x20e/0x400
[   41.840346]  v4l2_read+0x215/0x290
[   41.840603]  vfs_read+0x162/0x4c0

Fix this by checking the return value of cx88_risc_buffer()

[hverkuil: fix coding style issues]
Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/pci/cx88/cx88-vbi.c

@@ -144,11 +144,10 @@ static int buffer_prepare(struct vb2_buffer *vb)
 		return -EINVAL;
 	vb2_set_plane_payload(vb, 0, size);
 
-	cx88_risc_buffer(dev->pci, &buf->risc, sgt->sgl,
-			 0, VBI_LINE_LENGTH * lines,
-			 VBI_LINE_LENGTH, 0,
-			 lines);
-	return 0;
+	return cx88_risc_buffer(dev->pci, &buf->risc, sgt->sgl,
+				0, VBI_LINE_LENGTH * lines,
+				VBI_LINE_LENGTH, 0,
+				lines);
 }
 
 static void buffer_finish(struct vb2_buffer *vb)

drivers/media/pci/cx88/cx88-video.c

@@ -431,6 +431,7 @@ 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 cx8800_dev *dev = vb->vb2_queue->drv_priv;
 	struct cx88_core *core = dev->core;
@@ -445,35 +446,35 @@ static int buffer_prepare(struct vb2_buffer *vb)
 
 	switch (core->field) {
 	case V4L2_FIELD_TOP:
-		cx88_risc_buffer(dev->pci, &buf->risc,
-				 sgt->sgl, 0, UNSET,
-				 buf->bpl, 0, core->height);
+		ret = cx88_risc_buffer(dev->pci, &buf->risc,
+				       sgt->sgl, 0, UNSET,
+				       buf->bpl, 0, core->height);
 		break;
 	case V4L2_FIELD_BOTTOM:
-		cx88_risc_buffer(dev->pci, &buf->risc,
-				 sgt->sgl, UNSET, 0,
-				 buf->bpl, 0, core->height);
+		ret = cx88_risc_buffer(dev->pci, &buf->risc,
+				       sgt->sgl, UNSET, 0,
+				       buf->bpl, 0, core->height);
 		break;
 	case V4L2_FIELD_SEQ_TB:
-		cx88_risc_buffer(dev->pci, &buf->risc,
-				 sgt->sgl,
-				 0, buf->bpl * (core->height >> 1),
-				 buf->bpl, 0,
-				 core->height >> 1);
+		ret = cx88_risc_buffer(dev->pci, &buf->risc,
+				       sgt->sgl,
+				       0, buf->bpl * (core->height >> 1),
+				       buf->bpl, 0,
+				       core->height >> 1);
 		break;
 	case V4L2_FIELD_SEQ_BT:
-		cx88_risc_buffer(dev->pci, &buf->risc,
-				 sgt->sgl,
-				 buf->bpl * (core->height >> 1), 0,
-				 buf->bpl, 0,
-				 core->height >> 1);
+		ret = cx88_risc_buffer(dev->pci, &buf->risc,
+				       sgt->sgl,
+				       buf->bpl * (core->height >> 1), 0,
+				       buf->bpl, 0,
+				       core->height >> 1);
 		break;
 	case V4L2_FIELD_INTERLACED:
 	default:
-		cx88_risc_buffer(dev->pci, &buf->risc,
-				 sgt->sgl, 0, buf->bpl,
-				 buf->bpl, buf->bpl,
-				 core->height >> 1);
+		ret = cx88_risc_buffer(dev->pci, &buf->risc,
+				       sgt->sgl, 0, buf->bpl,
+				       buf->bpl, buf->bpl,
+				       core->height >> 1);
 		break;
 	}
 	dprintk(2,
@@ -481,7 +482,7 @@ static int buffer_prepare(struct vb2_buffer *vb)
 		buf, buf->vb.vb2_buf.index, __func__,
 		core->width, core->height, dev->fmt->depth, dev->fmt->fourcc,
 		(unsigned long)buf->risc.dma);
-	return 0;
+	return ret;
 }
 
 static void buffer_finish(struct vb2_buffer *vb)