linux-kernel - [PATCH -next 02/10] serial: core: Flush ldisc after dropping port mutex in uart

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Date:	Thu, 16 Oct 2014 16:54:19 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jiri Slaby <jslaby@...e.cz>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Peter Hurley <peter@...leysoftware.com>
Subject: [PATCH -next 02/10] serial: core: Flush ldisc after dropping port mutex in uart_close()

The tty buffers (and any line discipline buffers) must be flushed after
the UART hardware has shutdown; otherwise, a racing open on the same
tty may receive data from the previous session, which is a security
hazard. However, holding the port mutex while flushing the line
discipline buffers creates a lock inversion if the set_termios()
handler takes the port mutex (as it does in the followup patch,
'serial: Fix locking for uart driver set_termios method'.

Flush the ldisc buffers after dropping the port mutex; the tty lock
is still held which prevents a concurrent open() from advancing while
flushing. Since no new rx data is possible after uart_shutdown() until
a new open reinitializes the port, the later flush has no impact on
what data is being discarded.

Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
 drivers/tty/serial/serial_core.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index ce0e762..6203c6c 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1360,9 +1360,6 @@ static void uart_close(struct tty_struct *tty, struct file *filp)

 	mutex_lock(&port->mutex);
 	uart_shutdown(tty, state);
-
-	tty_ldisc_flush(tty);
-
 	tty_port_tty_set(port, NULL);
 	tty->closing = 0;
 	spin_lock_irqsave(&port->lock, flags);
@@ -1389,6 +1386,8 @@ static void uart_close(struct tty_struct *tty, struct file *filp)
 	wake_up_interruptible(&port->close_wait);

 	mutex_unlock(&port->mutex);
+
+	tty_ldisc_flush(tty);
 }

 static void uart_wait_until_sent(struct tty_struct *tty, int timeout)
-- 
2.1.1

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