linux-kernel - Re: [PATCH] tty: Add missing lock in n_tty

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <5196802A.604@hurleysoftware.com>
Date:	Fri, 17 May 2013 15:08:26 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Joerg Roedel <joro@...tes.org>
CC:	Jiri Slaby <jslaby@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: Add missing lock in n_tty_write()

On 05/17/2013 07:48 AM, Joerg Roedel wrote:
> Hi Peter,
>
> thanks for you explanations. They helped me to better understand what is
> happening now.
>
> On Wed, May 15, 2013 at 07:10:43PM -0400, Peter Hurley wrote:
>> On 05/15/2013 03:48 PM, Joerg Roedel wrote:
>
>> Agreed. Those functions look written for single-producer/single-consumer
>> i/o model. (That's why I asked about CONFIG_CONSOLE_POLL=y as well because
>> that doesn't look thread-safe either).
>
> Ok, I checked that. CONFIG_CONSOLE_POLL is on in that kernel.
>
>> Just to be clear here: there's a difference between a console driver
>> and a tty driver.
>>
>> The console driver's write() method is serialized with the global
>> console_lock() so parallel console writes are not possible.
>>
>> No such guarantee exists for the tty driver write() method, although it
>> probably wouldn't be difficult to provide that guarantee (since the
>> line discipline write() is already serialized by tty->atomic_write_lock).
>
> Okay, so it is safe to say that currently the drivers write() (and
> put_chars()) functions need to expect to be called concurrently and
> therefore they have to serialize themselves when they need it, right?

If only it were that simple :)

Yes, console write() and tty write() can be concurrent. However, the
console write() can also be _recursive_ wrt. tty write(). This can happen,
for example, if something oopses in the tty write() path.

If you review serial8250_console_write() in drivers/tty/serial/8250/8250_core.c,
you'll see how some of this is worked around.

But looking at this from a wider perspective, the goal should be
to limit the overlap as much as possible.

Regards,
Peter Hurley



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/