[PATCH] media: uvcvideo: Silence memcpy() run-time false positive warnings

[Kees Cook]

The memcpy() in uvc_video_decode_meta() intentionally copies across the
length and flags members and into the trailing buf flexible array.
Split the copy so that the compiler can better reason about (the lack
of) buffer overflows here. Avoid the run-time false positive warning:

  memcpy: detected field-spanning write (size 12) of single field "&meta->length" at drivers/media/usb/uvc/uvc_video.c:1355 (size 1)

Additionally fix a typo in the documentation for struct uvc_meta_buf.

Reported-by: ionut_n2001@xxxxxxxxx
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216810
Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
Cc: linux-media@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/media/usb/uvc/uvc_video.c | 4 +++-
 include/uapi/linux/uvcvideo.h     | 2 +-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index d2eb9066e4dc..b67347ab4181 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -1352,7 +1352,9 @@ static void uvc_video_decode_meta(struct uvc_streaming *stream,
 	if (has_scr)
 		memcpy(stream->clock.last_scr, scr, 6);
 
-	memcpy(&meta->length, mem, length);
+	meta->length = mem[0];
+	meta->flags  = mem[1];
+	memcpy(meta->buf, &mem[2], length - 2);
 	meta_buf->bytesused += length + sizeof(meta->ns) + sizeof(meta->sof);
 
 	uvc_dbg(stream->dev, FRAME,
diff --git a/include/uapi/linux/uvcvideo.h b/include/uapi/linux/uvcvideo.h
index 8288137387c0..a9d0a64007ba 100644
--- a/include/uapi/linux/uvcvideo.h
+++ b/include/uapi/linux/uvcvideo.h
@@ -86,7 +86,7 @@ struct uvc_xu_control_query {
  * struct. The first two fields are added by the driver, they can be used for
  * clock synchronisation. The rest is an exact copy of a UVC payload header.
  * Only complete objects with complete buffers are included. Therefore it's
- * always sizeof(meta->ts) + sizeof(meta->sof) + meta->length bytes large.
+ * always sizeof(meta->ns) + sizeof(meta->sof) + meta->length bytes large.
  */
 struct uvc_meta_buf {
 	__u64 ns;
-- 
2.34.1