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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sat, 20 Apr 2024 10:34:17 -0700
Message-ID: <CAHk-=wjEZvnn51dhhLqBKUd=cuFhbYA47_OyfUOPB-0zKToL7Q@mail.gmail.com>
Subject: Re: [PATCH v2] tty: n_gsm: restrict tty devices to attach
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jiri Slaby <jirislaby@kernel.org>, 
	Andrew Morton <akpm@linux-foundation.org>, "Starke, Daniel" <daniel.starke@siemens.com>, 
	LKML <linux-kernel@vger.kernel.org>, 
	linux-security-module <linux-security-module@vger.kernel.org>
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On Sat, 20 Apr 2024 at 04:12, Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> Since n_gsm is designed to be used for serial port [1], reject attaching to
> virtual consoles and PTY devices, by checking tty's device major/minor
> numbers at gsmld_open().

If we really just want to restrict it to serial devices, then do
something like, this:

   drivers/tty/n_gsm.c | 2 ++
   1 file changed, 2 insertions(+)

  diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
  index 4036566febcb..24425ef35b2b 100644
  --- a/drivers/tty/n_gsm.c
  +++ b/drivers/tty/n_gsm.c
  @@ -3629,6 +3629,8 @@ static int gsmld_open(struct tty_struct *tty)

        if (tty->ops->write == NULL)
                return -EINVAL;
  +     if (tty->ops->set_serial == NULL)
  +             return -EINVAL;

        /* Attach our ldisc data */
        gsm = gsm_alloc_mux();

which at least matches the current (largely useless) pattern of
checking for a write function.

I think all real serial sub-drivers already have that 'set_serial()'
function, and if there are some that don't, we could just add a dummy
for them. No?

Alternatively, we could go the opposite way, and have some flag in the
line discipline that says "I can be a console", and just check that in
tty_set_ldisc() for the console.

That would probably be a good idea regardless, but likely requires more effort.

But this kind of random major number testing seems wrong. It's trying
to deal with the _symptoms_, not some deeper truth.

                  Linus