linux-kernel - RE: [PATCH] tty: n_gsm: fix missing assignment of gsm->receive() in gsmld_attach

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Date:   Wed, 10 Aug 2022 09:08:13 +0000
From:   "Starke, Daniel" <daniel.starke@...mens.com>
To:     Mazin Al Haddad <mazinalhaddad05@...il.com>
CC:     "jirislaby@...nel.org" <jirislaby@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kernel-mentees@...ts.linuxfoundation.org" 
        <linux-kernel-mentees@...ts.linuxfoundation.org>,
        "skhan@...uxfoundation.org" <skhan@...uxfoundation.org>,
        "paskripkin@...il.com" <paskripkin@...il.com>,
        "syzbot+e3563f0c94e188366dbb@...kaller.appspotmail.com" 
        <syzbot+e3563f0c94e188366dbb@...kaller.appspotmail.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: RE: [PATCH] tty: n_gsm: fix missing assignment of gsm->receive() in
 gsmld_attach_gsm()

> Fix this by setting the gsm->receive() function when the line 
> discipline is being attached to the terminal device, inside 
> gsmld_attach_gsm(). This will guarantee that the function is assigned 
> and a call to TIOCSTI, which calls gsmld_receive_buf(), will not
> reference a null pointer.

In my opinion there are only two possible ways to fix this:
a) Move the gsm->receive initialization from gsm_activate_mux() to
gsmld_attach_gsm().
b) Avoid calling gsm->receive in gsmld_receive_buf() if not initialized.

The current code might assume that gsm->receive is only called after MUX
activation. Therefore, variant a) may break the code in other places.
I see no need to initialize gsm->receive in gsm_activate_mux() and
gsmld_attach_gsm().

Best regards,
Daniel Starke