um: fix ubd cow size

ubd_file_size() cannot use ubd_dev->cow.file because at this time
ubd_dev->cow.file is not initialized.
Therefore, ubd_file_size() will always report a wrong disk size when
COW files are used.
Reading from /dev/ubd* would crash the kernel.

We have to read the correct disk size from the COW file's backing
file.
Signed-off-by: Richard Weinberger <richard@nod.at>
CC: stable@kernel.org

arch/um/drivers/ubd_kern.c

@@ -511,8 +511,37 @@ __uml_exitcall(kill_io_thread);
 static inline int ubd_file_size(struct ubd *ubd_dev, __u64 *size_out)
 {
 	char *file;
+	int fd;
+	int err;
+
+	__u32 version;
+	__u32 align;
+	char *backing_file;
+	time_t mtime;
+	unsigned long long size;
+	int sector_size;
+	int bitmap_offset;
+
+	if (ubd_dev->file && ubd_dev->cow.file) {
+		file = ubd_dev->cow.file;
+
+		goto out;
+	}
 
-	file = ubd_dev->cow.file ? ubd_dev->cow.file : ubd_dev->file;
+	fd = os_open_file(ubd_dev->file, global_openflags, 0);
+	if (fd < 0)
+		return fd;
+
+	err = read_cow_header(file_reader, &fd, &version, &backing_file, \
+		&mtime, &size, &sector_size, &align, &bitmap_offset);
+	os_close_file(fd);
+
+	if(err == -EINVAL)
+		file = ubd_dev->file;
+	else
+		file = backing_file;
+
+out:
 	return os_file_size(file, size_out);
 }