[PATCH 5.4 054/102] tty: fix compat TIOCGSERIAL leaking uninitialized memory

[Greg Kroah-Hartman]

From: Eric Biggers <ebiggers@xxxxxxxxxx>

commit 17329563a97df3ba474eca5037c1336e46e14ff8 upstream.

Commit 77654350306a ("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: syzbot+8da9175e28eadcb203ce@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 77654350306a ("take compat TIOC[SG]SERIAL treatment into tty_compat_ioctl()")
Cc: <stable@xxxxxxxxxxxxxxx> # v4.20+
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Acked-by: Jiri Slaby <jslaby@xxxxxxx>
Link: https://lore.kernel.org/r/20200224182044.234553-2-ebiggers@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/tty/tty_io.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2731,7 +2731,9 @@ static int compat_tty_tiocgserial(struct
 	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;