Re: BUG: tty: memory corruption through tty_release/tty_ldisc_release

[Peter Hurley]

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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