[PATCH] virtio_console: take a kref in find_port_by_vq() to fix port UAF

[Hari Mishal <harimishal1@gmail.com>]

find_port_by_vq() returns a raw struct port pointer without taking a
reference on it, unlike find_port_by_devt_in_portdev() which does.
find_port_by_vq()'s only two callers, in_intr() and out_intr(), run as
virtqueue interrupt callbacks, entirely independent of and possibly
concurrently with unplug_port(), which itself runs from a workqueue when
the host sends a VIRTIO_CONSOLE_PORT_REMOVE control message.

unplug_port() removes the port from portdev->ports under ports_lock,
then later drops its last reference with kref_put(), freeing it via
remove_port(). find_port_by_vq() also walks portdev->ports under
ports_lock, so if it finds the port still on the list, the list removal,
and therefore the eventual kref_put(), has not happened yet, and taking
a reference at that point is always safe. Without doing so,
in_intr()/out_intr() can be left holding a pointer to a port that
unplug_port() frees on another core before they are done using it.

Both triggers are host-controlled as the host decides when to send the
PORT_REMOVE control message and when to kick the port's data vq. So a
malicious backend could race the two on purpose, without any guest side
cooperation. The freed object is a generic kmalloc allocation containing
a wait_queue_head_t, which in_intr()/out_intr() pass to
wake_up_interruptible() after touching the stale pointer.
wake_up_interruptible() invokes a function pointer read out of the wait
queue's entries. If the freed slab slot is reclaimed with attacker
influenced content before that call, then this is an arbitrary function
call primitive rather than just undefined behaviour.

Take a reference in find_port_by_vq() while still holding ports_lock,
matching find_port_by_devt_in_portdev(), and release it in in_intr() and
out_intr() once they are done with the port.

Signed-off-by: Hari Mishal <harimishal1@gmail.com>
---
 drivers/char/virtio_console.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index faef362dae85..1b63afe24e29 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -304,6 +304,12 @@ static struct port *find_port_by_id(struct ports_device *portdev, u32 id)
 	return port;
 }
 
+/*
+ * Finds a port by the virtqueue and returns a pointer to struct port
+ * with the reference count incremented.
+ *
+ * Callers MUST decrement it when finished.
+ */
 static struct port *find_port_by_vq(struct ports_device *portdev,
 				    struct virtqueue *vq)
 {
@@ -312,8 +318,10 @@ static struct port *find_port_by_vq(struct ports_device *portdev,
 
 	spin_lock_irqsave(&portdev->ports_lock, flags);
 	list_for_each_entry(port, &portdev->ports, list)
-		if (port->in_vq == vq || port->out_vq == vq)
+		if (port->in_vq == vq || port->out_vq == vq) {
+			kref_get(&port->kref);
 			goto out;
+		}
 	port = NULL;
 out:
 	spin_unlock_irqrestore(&portdev->ports_lock, flags);
@@ -1706,6 +1714,7 @@ static void out_intr(struct virtqueue *vq)
 	}
 
 	wake_up_interruptible(&port->waitqueue);
+	kref_put(&port->kref, remove_port);
 }
 
 static void in_intr(struct virtqueue *vq)
@@ -1723,6 +1732,7 @@ static void in_intr(struct virtqueue *vq)
 	if (!port->portdev) {
 		/* Port is being unplugged, ignore further data. */
 		spin_unlock_irqrestore(&port->inbuf_lock, flags);
+		kref_put(&port->kref, remove_port);
 		return;
 	}
 	port->inbuf = get_inbuf(port);
@@ -1756,6 +1766,8 @@ static void in_intr(struct virtqueue *vq)
 
 	if (is_console_port(port) && hvc_poll(port->cons.hvc))
 		hvc_kick();
+
+	kref_put(&port->kref, remove_port);
 }
 
 static void control_intr(struct virtqueue *vq)
-- 
2.43.0