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

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Message-ID: <5194E64A.3040003@ahsoftware.de>
Date:	Thu, 16 May 2013 15:59:38 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Peter Hurley <peter@...leysoftware.com>
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

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

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.

Regards,

Alexander Holler
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