linux-kernel - Re: [PATCH] tty: n_gsm: restrict tty devices to attach

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Date: Wed, 24 Jan 2024 05:14:33 -0800
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: Jiri Slaby <jirislaby@...nel.org>,
	syzbot <syzbot+06fa1063cca8163ea541@...kaller.appspotmail.com>,
	syzkaller-bugs@...glegroups.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Starke, Daniel" <daniel.starke@...mens.com>,
	Lee Jones <lee@...nel.org>, Fedor Pchelkin <pchelkin@...ras.ru>,
	linux-kernel@...r.kernel.org,
	linux-serial <linux-serial@...r.kernel.org>
Subject: Re: [PATCH] tty: n_gsm: restrict tty devices to attach

On Wed, Jan 24, 2024 at 07:06:04PM +0900, Tetsuo Handa wrote:
> syzbot is reporting sleep in atomic context, for gsmld_write() is calling
> con_write() with spinlock held and IRQs disabled.
> 
> Since n_gsm is designed to be used for serial port, reject attaching to
> virtual consoles and PTY devices, by checking tty's device major/minor
> numbers at gsmld_open().
> 
> Link: https://www.kernel.org/doc/html/v6.7/driver-api/tty/n_gsm.html
> Reported-by: syzbot+06fa1063cca8163ea541@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=06fa1063cca8163ea541
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> ---
>  drivers/tty/n_gsm.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index 4036566febcb..14581483af78 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -3623,6 +3623,7 @@ static void gsmld_close(struct tty_struct *tty)
>  static int gsmld_open(struct tty_struct *tty)
>  {
>  	struct gsm_mux *gsm;
> +	int major;
>  
>  	if (!capable(CAP_NET_ADMIN))
>  		return -EPERM;
> @@ -3630,6 +3631,17 @@ static int gsmld_open(struct tty_struct *tty)
>  	if (tty->ops->write == NULL)
>  		return -EINVAL;
>  
> +	major = tty->driver->major;
> +	/* Reject Virtual consoles */
> +	if (major == 4 && tty->driver->minor_start == 1)
> +		return -EINVAL;
> +	/* Reject Unix98 PTY masters/slaves */
> +	if (major >= 128 && major <= 143)
> +		return -EINVAL;
> +	/* Reject BSD PTY masters/slaves */
> +	if (major >= 2 && major <= 3)
> +		return -EINVAL;

That is a lot of hard-coded magic numbers, why aren't these defined
anywhere?

But really, this is only for fuzz testers, why can't they just not ever
bind this to a console?  Why do we have to have these checks in the
kernel to prevent userspace from doing dumb things that no real
userspace will ever do?

thanks,

greg k-h