virtio_console: Introduce an ID allocator for virtual console numbers

When a virtio console port is initialized, it is registered as an hvc
console using a virtual console number. If a KVM guest is started with
multiple virtio console devices, the same vtermno (or virtual console
number) can be used to allocate different hvc consoles, which leads to
various communication problems later on.

This is also reported in debugfs :

  # grep vtermno /sys/kernel/debug/virtio-ports/*
  /sys/kernel/debug/virtio-ports/vport1p1:console_vtermno: 1
  /sys/kernel/debug/virtio-ports/vport2p1:console_vtermno: 1
  /sys/kernel/debug/virtio-ports/vport3p1:console_vtermno: 2
  /sys/kernel/debug/virtio-ports/vport4p1:console_vtermno: 3

Replace the next_vtermno global with an ID allocator and start the
allocation at 1 as it is today. Also recycle IDs when a console port
is removed.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20221122134643.376184-1-clg@kaod.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/virtio_console.c

@@ -13,6 +13,7 @@
 #include <linux/fs.h>
 #include <linux/splice.h>
 #include <linux/pagemap.h>
+#include <linux/idr.h>
 #include <linux/init.h>
 #include <linux/list.h>
 #include <linux/poll.h>
@@ -48,22 +49,11 @@ struct ports_driver_data {
 	/* List of all the devices we're handling */
 	struct list_head portdevs;
 
-	/*
-	 * This is used to keep track of the number of hvc consoles
-	 * spawned by this driver.  This number is given as the first
-	 * argument to hvc_alloc().  To correctly map an initial
-	 * console spawned via hvc_instantiate to the console being
-	 * hooked up via hvc_alloc, we need to pass the same vtermno.
-	 *
-	 * We also just assume the first console being initialised was
-	 * the first one that got used as the initial console.
-	 */
-	unsigned int next_vtermno;
-
 	/* All the console devices handled by this driver */
 	struct list_head consoles;
 };
-static struct ports_driver_data pdrvdata = { .next_vtermno = 1};
+
+static struct ports_driver_data pdrvdata;
 
 static DEFINE_SPINLOCK(pdrvdata_lock);
 static DECLARE_COMPLETION(early_console_added);
@@ -89,6 +79,8 @@ struct console {
 	u32 vtermno;
 };
 
+static DEFINE_IDA(vtermno_ida);
+
 struct port_buffer {
 	char *buf;
 
@@ -1244,18 +1236,21 @@ static int init_port_console(struct port *port)
 	 * pointers.  The final argument is the output buffer size: we
 	 * can do any size, so we put PAGE_SIZE here.
 	 */
-	port->cons.vtermno = pdrvdata.next_vtermno;
+	ret = ida_alloc_min(&vtermno_ida, 1, GFP_KERNEL);
+	if (ret < 0)
+		return ret;
 
+	port->cons.vtermno = ret;
 	port->cons.hvc = hvc_alloc(port->cons.vtermno, 0, &hv_ops, PAGE_SIZE);
 	if (IS_ERR(port->cons.hvc)) {
 		ret = PTR_ERR(port->cons.hvc);
 		dev_err(port->dev,
 			"error %d allocating hvc for port\n", ret);
 		port->cons.hvc = NULL;
+		ida_free(&vtermno_ida, port->cons.vtermno);
 		return ret;
 	}
 	spin_lock_irq(&pdrvdata_lock);
-	pdrvdata.next_vtermno++;
 	list_add_tail(&port->cons.list, &pdrvdata.consoles);
 	spin_unlock_irq(&pdrvdata_lock);
 	port->guest_connected = true;
@@ -1532,6 +1527,7 @@ static void unplug_port(struct port *port)
 		list_del(&port->cons.list);
 		spin_unlock_irq(&pdrvdata_lock);
 		hvc_remove(port->cons.hvc);
+		ida_free(&vtermno_ida, port->cons.vtermno);
 	}
 
 	remove_port_data(port);