linux-kernel - Re: [PATCH] tty_buffer: avoid race due to tty_buffer_free

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Message-ID: <5194D6F9.5000505@hurleysoftware.com>
Date:	Thu, 16 May 2013 08:54:17 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	channing <chao.bi@...el.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty_buffer: avoid race due to tty_buffer_free_all() being
 misused

On 05/16/2013 04:59 AM, channing wrote:
>
> In tty_buffer.c, function tty_buffer_free_all() is used to remove
> all buffers for a tty, although it's declared that it mustn't be called
> when the tty is in use, it cannot guarantee that. we can observe some
> device driver make use it by mistake, for example, while tty device is
> releasing, the tty data forwarding is not stopped, then it might hit
> the case that tty buffer is being used while tty_buffer_free_all()
> free this tty buffer, and finally lead to random error at any places,
> and it's not clear to debug.

What kernel version?

> Although device driver could do better, it's simpler and safer to
> strengthen protection in the view of tty buffer, by adding a tty->buf.lock
> in tty_buffer_free_all() to avoid it racing with ongoing tty buffer
> operations.

Sorry, but this isn't correct.

The driver cannot continue to perform i/o concurrently with
tty_port_destroy().

If the concurrent use you're observing is with flush_to_ldisc(),
that should be fixed in current mainline.

Regards,
Peter Hurley
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