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

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.

CC: <stable@vger.kernel.org>
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>

1 files changed, 6 insertions, 7 deletions

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 8a15af3e1a9d..3beea9d478bc 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -272,9 +272,12 @@ 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);
@@ -1036,14 +1039,10 @@ 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