linux-kernel - Re: [PATCH] tty: make sure a BUG is hit if tty

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Message-ID: <5196B477.8080904@hurleysoftware.com>
Date:	Fri, 17 May 2013 18:51:35 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Alexander Holler <holler@...oftware.de>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] tty: make sure a BUG is hit if tty_port will be destroyed
 before tty

On 05/17/2013 03:43 PM, Alexander Holler wrote:
> Am 17.05.2013 21:22, schrieb Alexander Holler:
>
>> The case that the machine didn't die, but just the process, only happens
>> when my proposed patch is applied, which prevents the memory corruption.
>
> In short, the proposed BUG_ON() prevents the memory corruption because
> it is hit before something bad can happen. The result is that just the
> process in question will be killed (and a tty is not released), but only
> that BUG_ON() prevents that something _really_ bad happens.
>
> I hope I could describe it now clearly. ;)

Your descriptions have been clear and I understood your meaning. However,
I think you may have misunderstood my suggestion.

Would you please test the patch below?

--- >% ---
Subject: [PATCH] tty: Prevent tty_port destruction if tty not released

If the tty driver mistakenly drops the last port reference
before the tty has been released, issue a diagnostic and
abort the port destruction.

This will leak memory and may zombify the port, but should
otherwise keep the machine in runnable state.

Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
  drivers/tty/tty_port.c | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index 6d9e0b2..a4f4fa9 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -140,6 +140,10 @@ EXPORT_SYMBOL(tty_port_destroy);
  static void tty_port_destructor(struct kref *kref)
  {
  	struct tty_port *port = container_of(kref, struct tty_port, kref);
+
+	/* check if last port ref was dropped before tty release */
+	if (WARN_ON(port->itty))
+		return;
  	if (port->xmit_buf)
  		free_page((unsigned long)port->xmit_buf);
  	tty_port_destroy(port);
-- 
1.8.1.2


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