linux-kernel - Re: [PATCH] tty: fix data race in n_tty_receive_buf

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Message-ID: <20180105141504.0e399394@alans-desktop>
Date:   Fri, 5 Jan 2018 14:15:04 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     "Kohli, Gaurav" <gkohli@...eaurora.org>
Cc:     jslaby@...e.com, gregkh@...uxfoundation.org, mikey@...ling.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] tty: fix data race in n_tty_receive_buf_common

> But in above case , there we can hit another race, if we have a sequence 
> like this
> tty_init_dev->alloc_tty_struct -> tty_ldisc_init -> this will initialize 
> ldisc ,
> but at this moment disc_data is still NULL
> 
> And if flush_to_ldisc comes in between, it will take ldisc reference and 
> proceeds receive buffer.

So you need to move the lock up one line to protect the assignment to
tty->port->itty. We can do that.

At that point your flush_to_ldisc should see either port->itty = NULL or a
valid initialized ldisc.

Alan