virtio: console: fix race with port unplug and open/close

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@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index d5ae736..5746a36 100644 (file)
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -257,9 +257,12 @@ static struct port *find_port_by_devt_in_portdev(struct ports_device *portdev,
        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 *inode, struct file *filp)
        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