linux-kernel - Re: BUG: tty: memory corruption through tty_release/tty_ldisc

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Message-ID: <5195553C.90608@hurleysoftware.com>
Date:	Thu, 16 May 2013 17:53:00 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Alexander Holler <holler@...oftware.de>
CC:	linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Marcel Holtmann <marcel@...tmann.org>,
	Gustavo Padovan <gustavo@...ovan.org>,
	Johan Hedberg <johan.hedberg@...il.com>,
	linux-bluetooth@...r.kernel.org
Subject: Re: BUG: tty: memory corruption through tty_release/tty_ldisc_release

On 05/16/2013 09:59 AM, Alexander Holler wrote:
> Am 16.05.2013 15:47, schrieb Peter Hurley:
>> On 05/16/2013 02:45 AM, Alexander Holler wrote:
>>> Hello,
>>>
>>> after some pain because the "big step" (ecbbfd4) happened while the
>>> support for my AMD CPU was broken and thus git bisect hit a series of
>>> kernels which didn't boot, I've finally found the cause for a memory
>>> corruption: tty_ldisc_release().
>>>
>>> What happens is the following:
>>>
>>> tty_port is self-destructing, that means it destroys itself in
>>> tty_port.c:tty_port_destructor() when the last reference is gone. E.g.
>>> in case of rfcomm this happens with the call to tty->ops->close() in
>>> tty_io.c:tty_release().
>>>
>>> The problem here is that tty_io.c:tty_release() calls
>>> tty_ldisc.c:tty_ldisc_release() which uses the tty_port to flush the
>>> ldisc work queues.
>>>
>>> In the best case this hits a BUG() in cancel_work_sync() but often it
>>> just causes a memory corruption without a BUG() got hit before.
>>
>> Hi Alexander,
>>
>> Actually, the problem is that tty->ops->close() shouldn't be
>> the last kref on the port.
>>
>> It doesn't look to me like device removal is being handled
>> properly.
>>
>
> Maybe, but if so, that should be documented (and ideally prevented).

The tty_port documentation is trapped in the place as _all_ the
bluetooth documentation :)

And the tty layer can't really _prevent_ the tty driver from mishandling
the port kref.

> Especially since it seemed to have been worked before tty_ports got introduced.

Well, at the time tty_port was introduced to RFCOMM, there was nothing
to tear-down in tty_port. Now that tty_port owns the flip buffers and
must do proper tear-down, the problem has surfaced.

> But I can't add much more to this discussion, as I'm rather a novice in regard to the tty subsystem. I even don't know much about the task sharing between tty, tty_port and tty_ldisc, except the stuff I found out because I got hit by that bug and therefor have read some of the sources.

Ok. Could you paste the BUG() and steps to reproduce?
I have a plan to fix it but I'd like to review what you have
first.

Regards,
Peter Hurley

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