media: usb: pwc-uncompress: Use flex array destination for memcpy()

In preparation for FORTIFY_SOURCE performing run-time destination buffer
bounds checking for memcpy(), specify the destination output buffer
explicitly, instead of asking memcpy() to write past the end of what looked
like a fixed-size object.

Notice that raw_frame is a pointer to a structure that contains
flexible-array member rawframe[]:

drivers/media/usb/pwc/pwc.h:
190 struct pwc_raw_frame {
191         __le16 type;            /* type of the webcam */
192         __le16 vbandlength;     /* Size of 4 lines compressed (used by the
193                                    decompressor) */
194         __u8   cmd[4];          /* the four byte of the command (in case of
195                                    nala, only the first 3 bytes is filled) */
196         __u8   rawframe[];      /* frame_size = H / 4 * vbandlength */
197 } __packed;

Link: https://github.com/KSPP/linux/issues/200Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

drivers/media/usb/pwc/pwc-uncompress.c

@@ -39,7 +39,7 @@ int pwc_decompress(struct pwc_device *pdev, struct pwc_frame_buf *fbuf)
 			 * first 3 bytes is filled (Nala case). We can
 			 * determine this using the type of the webcam */
 		memcpy(raw_frame->cmd, pdev->cmd_buf, 4);
-		memcpy(raw_frame+1, yuv, pdev->frame_size);
+		memcpy(raw_frame->rawframe, yuv, pdev->frame_size);
 		vb2_set_plane_payload(&fbuf->vb.vb2_buf, 0,
 			struct_size(raw_frame, rawframe, pdev->frame_size));
 		return 0;