linux-kernel - [043/121] virtio: console: fix race with port unplug and open/close

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Date:	Sun, 08 Sep 2013 03:52:01 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Rusty Russell" <rusty@...tcorp.com.au>,
	"Mateusz Guzik" <mguzik@...hat.com>,
	"Amit Shah" <amit.shah@...hat.com>
Subject: [043/121] virtio: console: fix race with port unplug and open/close

3.2.51-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Amit Shah <amit.shah@...hat.com>

commit 057b82be3ca3d066478e43b162fc082930a746c9 upstream.

There's a window between find_port_by_devt() returning a port and us
taking a kref on the port, where the port could get unplugged.  Fix it
by taking the reference in find_port_by_devt() itself.

Problem reported and analyzed by Mateusz Guzik.

Reported-by: Mateusz Guzik <mguzik@...hat.com>
Signed-off-by: Amit Shah <amit.shah@...hat.com>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/char/virtio_console.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -257,9 +257,12 @@ static struct port *find_port_by_devt_in
 	unsigned long flags;
 
 	spin_lock_irqsave(&portdev->ports_lock, flags);
-	list_for_each_entry(port, &portdev->ports, list)
-		if (port->cdev->dev == dev)
+	list_for_each_entry(port, &portdev->ports, list) {
+		if (port->cdev->dev == dev) {
+			kref_get(&port->kref);
 			goto out;
+		}
+	}
 	port = NULL;
 out:
 	spin_unlock_irqrestore(&portdev->ports_lock, flags);
@@ -793,14 +796,10 @@ static int port_fops_open(struct inode *
 	struct port *port;
 	int ret;
 
+	/* We get the port with a kref here */
 	port = find_port_by_devt(cdev->dev);
 	filp->private_data = port;
 
-	/* Prevent against a port getting hot-unplugged at the same time */
-	spin_lock_irq(&port->portdev->ports_lock);
-	kref_get(&port->kref);
-	spin_unlock_irq(&port->portdev->ports_lock);
-
 	/*
 	 * Don't allow opening of console port devices -- that's done
 	 * via /dev/hvc

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