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    Subject[PATCH 1/1] usb: cdc-acm: Fix acm_tty_hangup() vs. acm_tty_close() race
    Date
    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>
    ---
    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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