linux-kernel - [PATCH 05/24] Bluetooth: Fix racy acquire of rfcomm

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Message-Id: <1391997564-1805-6-git-send-email-peter@hurleysoftware.com>
Date:	Sun,  9 Feb 2014 20:59:05 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Marcel Holtmann <marcel@...tmann.org>
Cc:	Gustavo Padovan <gustavo@...ovan.org>,
	Johan Hedberg <johan.hedberg@...il.com>,
	Gianluca Anzolin <gianluca@...tospazio.it>,
	Alexander Holler <holler@...oftware.de>,
	Andrey Vihrov <andrey.vihrov@...il.com>,
	Sander Eikelenboom <linux@...elenboom.it>,
	linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Hurley <peter@...leysoftware.com>,
	Jiri Slaby <jslaby@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH 05/24] Bluetooth: Fix racy acquire of rfcomm_dev reference

rfcomm_dev_get() can return a rfcomm_dev reference for a
device for which destruction may be commencing. This can happen
on tty destruction, which calls rfcomm_tty_cleanup(), the last
port reference may have been released but RFCOMM_TTY_RELEASED
was not set. The following race is also possible:

CPU 0                            | CPU 1
                                 | rfcomm_release_dev
rfcomm_dev_get                   |   .
  spin_lock                      |   .
    dev  = __rfcomm_dev_get      |   .
    if dev                       |   .
      if test_bit(TTY_RELEASED)  |   .
                                 |   !test_and_set_bit(TTY_RELEASED)
                                 |     tty_port_put   <<<< last reference
      else                       |
        tty_port_get             |

The reference acquire is bogus because destruction will commence
with the release of the last reference.

Ignore the external state change of TTY_RELEASED and instead rely
on the reference acquire itself to determine if the reference is
valid.

Cc: Jiri Slaby <jslaby@...e.cz>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
 net/bluetooth/rfcomm/tty.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/net/bluetooth/rfcomm/tty.c b/net/bluetooth/rfcomm/tty.c
index a535ef1..7cf193f 100644
--- a/net/bluetooth/rfcomm/tty.c
+++ b/net/bluetooth/rfcomm/tty.c
@@ -157,12 +157,8 @@ static struct rfcomm_dev *rfcomm_dev_get(int id)
 
 	dev = __rfcomm_dev_get(id);
 
-	if (dev) {
-		if (test_bit(RFCOMM_TTY_RELEASED, &dev->flags))
-			dev = NULL;
-		else
-			tty_port_get(&dev->port);
-	}
+	if (dev && !tty_port_get(&dev->port))
+		dev = NULL;
 
 	spin_unlock(&rfcomm_dev_lock);
 
-- 
1.8.1.2

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