linux-kernel - Re: [PATCH v2 2/2] tty: use tty_init_dev_retry() to workaround a race condition

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Date:   Thu, 21 Nov 2019 21:01:55 +0000
From:   Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jslaby@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] tty: use tty_init_dev_retry() to workaround a
 race condition

Hi Greg,

On Thu, Nov 21, 2019 at 05:41:38PM +0100, Greg Kroah-Hartman wrote:
> On Thu, Nov 21, 2019 at 03:22:39PM +0000, Sudip Mukherjee wrote:
> > There seems to be a race condition in tty drivers and I could see on
> > many boot cycles a NULL pointer dereference as tty_init_dev() tries to
> > do 'tty->port->itty = tty' even though tty->port is NULL.
> > 'tty->port' will be set by the driver and if the driver has not yet done
> > it before we open the tty device we can get to this situation. By adding
> > some extra debug prints, I noticed that:
> > 
> > 6.650130: uart_add_one_port
> > 6.663849: register_console
> > 6.664846: tty_open
> > 6.674391: tty_init_dev
> > 6.675456: tty_port_link_device
> > 
> > uart_add_one_port() registers the console, as soon as it registers, the
> > userspace tries to use it and that leads to tty_open() but
> > uart_add_one_port() has not yet done tty_port_link_device() and so
> > tty->port is not yet configured when control reaches tty_init_dev().
> 
> Shouldn't we do tty_port_link_device() before uart_add_one_port() to
> remove that race?  Once you register the console, yes, tty_open() can
> happen, so the driver had better be ready to go at that point in time.
> 

But tty_port_link_device() is done by uart_add_one_port() itself.
After registering the console uart_add_one_port() will call
tty_port_register_device_attr_serdev() and tty_port_link_device() is
called from this. Thats still tty core.

> This feels like it should be fixed by the caller, not in the tty core.
> Any reason that can not happen?

tty_port_register_device_attr_serdev() is part of tty core. Or is my
above understanding wrong?


--
Regards
Sudip