Re: [PATCH] tty: vt: hold tty reference for keyboard callbacks

[Jiri Slaby]

On 02. 06. 26, 8:15, Morduan Zang wrote:
> From: Zhan Jun <zhanjun@xxxxxxxxxxxxx>
>
> syzbot reported a use-after-free in stop_tty() when the VT
> keyboard path handles the hold key.
>
> The keyboard event path reads vc->port.tty under kbd_event_lock,
> but con_shutdown() clears the pointer under console_lock and the tty
> can be released after the final close. The keyboard lock therefore
> does not protect the tty lifetime.
>
> Let the VT port own a tty reference by using tty_port_tty_set() when
> installing and shutting down the console tty. Use tty_port_tty_get()
> in the keyboard paths before dereferencing vc->port.tty and drop the
> reference after the last use.
>
> Reported-by: syzbot+2932e8970a6398db95c3@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/6a1dde0d.bd48a97d.14881d.0005.GAE@xxxxxxxxxx/
> Signed-off-by: Zhan Jun <zhanjun@xxxxxxxxxxxxx>
> ---
>   drivers/tty/vt/keyboard.c | 17 ++++++++++++-----
>   drivers/tty/vt/vt.c       |  4 ++--
>   2 files changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
> index dfdea0842149..19f8df9706ee 100644
> --- a/drivers/tty/vt/keyboard.c
> +++ b/drivers/tty/vt/keyboard.c
> @@ -509,9 +509,13 @@ static void fn_show_ptregs(struct vc_data *vc)
>   
>   static void fn_hold(struct vc_data *vc)
>   {
> -	struct tty_struct *tty = vc->port.tty;
> +	struct tty_struct *tty;
> +
> +	if (rep)
> +		return;
>   
> -	if (rep || !tty)
> +	tty = tty_port_tty_get(&vc->port);

We have guards (tty_port_tty) and you should use those.

thanks,
--
js
suse labs