tty: fix compat TIOCGSERIAL leaking uninitialized memory

Commit 7765435 ("take compat TIOC[SG]SERIAL treatment into
tty_compat_ioctl()") changed the compat version of TIOCGSERIAL to start
copying a whole 'serial_struct32' to userspace rather than individual
fields, but failed to initialize all padding and fields -- namely the
hole after the 'iomem_reg_shift' field, and the 'reserved' field.

Fix this by initializing the struct to zero.

[v2: use sizeof, and convert the adjacent line for consistency.]

Reported-by: [email protected]
Fixes: 7765435 ("take compat TIOC[SG]SERIAL treatment into tty_compat_ioctl()")
Cc: <[email protected]> # v4.20+
Signed-off-by: Eric Biggers <[email protected]>
Acked-by: Jiri Slaby <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: ebiggers

drivers/tty/tty_io.c

@@ -2730,7 +2730,9 @@ static int compat_tty_tiocgserial(struct tty_struct *tty,
 	struct serial_struct32 v32;
 	struct serial_struct v;
 	int err;
-	memset(&v, 0, sizeof(struct serial_struct));
+
+	memset(&v, 0, sizeof(v));
+	memset(&v32, 0, sizeof(v32));
 
 	if (!tty->ops->set_serial)
 		return -ENOTTY;