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Subject[PATCH] tty: fix ldisc flush and termios setting race
From: Min Zhang <mzhang@mvista.com>

A race condition can clear tty ldisc icanon bit unintentionally which
could stop n_tty from processing received characters. It can occur when
tty receiver buffer was full, e.g. 4096 chars received, 8250 serial driver
interrupt tried to flush_to_ldisc them, but other shell thread tried
to change_termios the tty setting too. Then n_tty_receive_char and
n_tty_set_termios both tried to modify n_tty_data fields.

Specifically the icanon and its neighbor fields are defines as bits:

unsigned char lnext:1, erasing:1, raw:1, real_raw:1, icanon:1;

However they are not atomically accessed in the follow race condition:

serial8250_handle_irq
serail8250_rx_chars
tty_flip_buffer_push
schdule_work -------> flush_to_ldisc
n_tty_receive_buf
n_tty_receive_char
(holds no lock)
ioctl
set_termios
tty_set_termios
n_tty_set_termios
(holds termios_mutex)
The patch let change_termios to use TTY_LDISC_FLUSHING to prevent
flush_to_ldisc from running, and flush_to_ldisc use TTY_FLUSHING
to make change_termios wait until the flag is cleared.

The patch also replaces flush_to_ldisc's wake_up with wake_up_all,
because it can now wake up both TTY_FLUSHING and TTY_FLUSHPENDING
on the same tty->read_wait queue. Event waiters need check which event.

Signed-off-by: Min Zhang <mzhang@mvista.com>
---
drivers/tty/tty_buffer.c | 12 +++++++++++-
drivers/tty/tty_ioctl.c | 27 ++++++++++++++++++++++++++-
2 files changed, 37 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 45d9161..37f4818 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -482,6 +482,13 @@ static void flush_to_ldisc(struct work_struct *work)

spin_lock_irqsave(&buf->lock, flags);

+ /* Ldisc change by change_termios can race with ldisc receive_buf, esp
+ to access N_TTY line discipline field in tty_struct, so we defer */
+ if (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
+ schedule_delayed_work(&buf->work, 1);
+ goto out;
+ }
+
if (!test_and_set_bit(TTYP_FLUSHING, &port->iflags)) {
struct tty_buffer *head;
while ((head = buf->head) != NULL) {
@@ -522,8 +529,11 @@ static void flush_to_ldisc(struct work_struct *work)
if (test_bit(TTYP_FLUSHPENDING, &port->iflags)) {
__tty_buffer_flush(port);
clear_bit(TTYP_FLUSHPENDING, &port->iflags);
- wake_up(&tty->read_wait);
}
+
+ /* wake up tasks waiting for TTYP_FLUSHING or TTYP_FLUSHPENDING */
+ wake_up_all(&tty->read_wait);
+out:
spin_unlock_irqrestore(&buf->lock, flags);

tty_ldisc_deref(disc);
diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
index 8481b29..36a1bfa 100644
--- a/drivers/tty/tty_ioctl.c
+++ b/drivers/tty/tty_ioctl.c
@@ -546,8 +546,33 @@ int tty_set_termios(struct tty_struct *tty, struct ktermios *new_termios)

ld = tty_ldisc_ref(tty);
if (ld != NULL) {
- if (ld->ops->set_termios)
+ unsigned long flags;
+ struct tty_port *port = tty->port;
+ struct tty_bufhead *buf = &port->buf;
+ if (!ld->ops->set_termios)
+ goto no_set_termios;
+
+ /* Wait if the data is being pushed to the tty layer */
+ spin_lock_irqsave(&buf->lock, flags);
+ while (test_bit(TTYP_FLUSHING, &port->iflags)) {
+ spin_unlock_irqrestore(&buf->lock, flags);
+ printk(KERN_CRIT "%s flushing\n", __FUNCTION__);
+ wait_event(tty->read_wait,
+ test_bit(TTYP_FLUSHING, &port->iflags) == 0);
+ spin_lock_irqsave(&buf->lock, flags);
+ }
+ printk(KERN_CRIT "%s survived flushing\n", __FUNCTION__);
+
+ /* Prevent future flush_to_ldisc while we are setting */
+ if (!test_and_set_bit(TTY_LDISC_CHANGING, &tty->flags)) {
+ spin_unlock_irqrestore(&buf->lock, flags);
(ld->ops->set_termios)(tty, &old_termios);
+ spin_lock_irqsave(&buf->lock, flags);
+ clear_bit(TTY_LDISC_CHANGING, &tty->flags);
+ }
+ spin_unlock_irqrestore(&buf->lock, flags);
+
+no_set_termios:
tty_ldisc_deref(ld);
}
mutex_unlock(&tty->termios_mutex);
--
1.7.7.4


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