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Subject[48/49] usb: cdc-acm: Fix acm_tty_hangup() vs. acm_tty_close() race
3.2-stable review patch.  If anyone has any objections, please let me know.

------------------
From: Thilo-Alexander Ginkel <thilo@ginkel.com>

[Not upstream as it was fixed differently for 3.3 with a much more
"intrusive" rework of the driver - gregkh]

There is a race condition involving acm_tty_hangup() and acm_tty_close()
where hangup() would attempt to access tty->driver_data without proper
locking and NULL checking after close() has potentially already set it
to NULL. One possibility to (sporadically) trigger this behavior is to
perform a suspend/resume cycle with a running WWAN data connection.

This patch addresses the issue by introducing a NULL check for
tty->driver_data in acm_tty_hangup() protected by open_mutex and exiting
gracefully when hangup() is invoked on a device that has already been
closed.

Signed-off-by: Thilo-Alexander Ginkel <thilo@ginkel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
drivers/usb/class/cdc-acm.c | 12 ++++++++++--
1 files changed, 10 insertions(+), 2 deletions(-)
diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index a8078d0..97f2e58 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -554,10 +554,18 @@ static void acm_port_down(struct acm *acm)

static void acm_tty_hangup(struct tty_struct *tty)
{
- struct acm *acm = tty->driver_data;
- tty_port_hangup(&acm->port);
+ struct acm *acm;
+
mutex_lock(&open_mutex);
+ acm = tty->driver_data;
+
+ if (!acm)
+ goto out;
+
+ tty_port_hangup(&acm->port);
acm_port_down(acm);
+
+out:
mutex_unlock(&open_mutex);
}

--
1.7.5.4
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