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Subject[PATCHv5 05/17] tty: don't allow ldisc dcd_change() after ldisc halt
Date
There was a possibility that uart_handle_dcd_change() could obtain a
reference to ldisc while running in parallel with tty_set_ldisc() on
different CPU but call dcd_change() operation after tty_ldisc_close()
which is incorrect.

Treat this situation specially by locking the whole
uart_handle_dcd_change() with spinlock and adding a "barrier" to
tty_ldisc_halt() which ensures that there are no active ldisc
references in uart_handle_dcd_change() after tty_ldisc_halt().

Signed-off-by: Alexander Gordeev <lasaine@lvk.cs.msu.su>
---
drivers/char/tty_io.c | 1 +
drivers/char/tty_ldisc.c | 7 +++++++
include/linux/serial_core.h | 18 ++++++++++++------
include/linux/tty.h | 1 +
4 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 613c852..18576d4 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -2816,6 +2816,7 @@ void initialize_tty_struct(struct tty_struct *tty,
mutex_init(&tty->echo_lock);
spin_lock_init(&tty->read_lock);
spin_lock_init(&tty->ctrl_lock);
+ spin_lock_init(&tty->dcd_change_lock);
INIT_LIST_HEAD(&tty->tty_files);
INIT_WORK(&tty->SAK_work, do_SAK_work);

diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index 412f977..27fadb0 100644
--- a/drivers/char/tty_ldisc.c
+++ b/drivers/char/tty_ldisc.c
@@ -522,11 +522,18 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
* You need to do a 'flush_scheduled_work()' (outside the ldisc_mutex)
* in order to make sure any currently executing ldisc work is also
* flushed.
+ *
+ * dcd_change() doesn't use workqueues so it needs a special
+ * "barrier", which ensures that there are no active ldisc references
+ * in dcd_change().
*/

static int tty_ldisc_halt(struct tty_struct *tty)
{
+ spin_lock_irq(&tty->dcd_change_lock);
clear_bit(TTY_LDISC, &tty->flags);
+ spin_unlock_irq(&tty->dcd_change_lock);
+
return cancel_delayed_work_sync(&tty->buf.work);
}

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 55c8192..62835b6 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -508,11 +508,15 @@ static inline int uart_handle_break(struct uart_port *port)
static inline void
uart_handle_dcd_change(struct uart_port *uport, unsigned int status)
{
- struct uart_state *state = uport->state;
- struct tty_port *port = &state->port;
- struct tty_ldisc *ld = tty_ldisc_ref(port->tty);
+ struct tty_port *port = &uport->state->port;
+ struct tty_struct *tty = port->tty;
struct pps_event_time ts;
+ struct tty_ldisc *ld;
+ unsigned long flags;

+ spin_lock_irqsave(&tty->dcd_change_lock, flags);
+
+ ld = tty_ldisc_ref(tty);
if (ld && ld->ops->dcd_change)
pps_get_ts(&ts);

@@ -525,14 +529,16 @@ uart_handle_dcd_change(struct uart_port *uport, unsigned int status)
if (port->flags & ASYNC_CHECK_CD) {
if (status)
wake_up_interruptible(&port->open_wait);
- else if (port->tty)
- tty_hangup(port->tty);
+ else if (tty)
+ tty_hangup(tty);
}

if (ld && ld->ops->dcd_change)
- ld->ops->dcd_change(port->tty, status, &ts);
+ ld->ops->dcd_change(tty, status, &ts);
if (ld)
tty_ldisc_deref(ld);
+
+ spin_unlock_irqrestore(&tty->dcd_change_lock, flags);
}

/**
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 67d64e6..506fe1c 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -327,6 +327,7 @@ struct tty_struct {
/* If the tty has a pending do_SAK, queue it here - akpm */
struct work_struct SAK_work;
struct tty_port *port;
+ spinlock_t dcd_change_lock;
};

/* Each of a tty's open files has private_data pointing to tty_file_private */
--
1.7.2.3


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