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

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Message-ID: <eef58b47-f208-2ac5-6e02-a87f9568c70f@suse.com>
Date:   Fri, 22 Nov 2019 10:05:09 +0100
From:   Jiri Slaby <jslaby@...e.com>
To:     Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] tty: use tty_init_dev_retry() to workaround a race
 condition

On 21. 11. 19, 22:01, Sudip Mukherjee wrote:
> 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.

Interferences of console vs tty code are ugly. Does it help to simply
put tty_port_link_device to uart_add_one_port before uart_configure_port?

thanks,
-- 
js
suse labs