linux-kernel - [PATCH] virtio_console: Use an atomic to allocate virtual console numbers

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Message-Id: <20221114080752.1900699-1-clg@kaod.org>
Date:   Mon, 14 Nov 2022 09:07:52 +0100
From:   Cédric Le Goater <clg@...d.org>
To:     Amit Shah <amit@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        Cédric Le Goater <clg@...d.org>
Subject: [PATCH] virtio_console: Use an atomic to allocate 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

Fix the issue with an atomic variable and start the first console
number at 1 as it is today.

Signed-off-by: Cédric Le Goater <clg@...d.org>
---
 drivers/char/virtio_console.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 9fa3c76a267f..253574f41e57 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -58,12 +58,13 @@ struct ports_driver_data {
 	 * We also just assume the first console being initialised was
 	 * the first one that got used as the initial console.
 	 */
-	unsigned int next_vtermno;
+	atomic_t 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 = { .next_vtermno = ATOMIC_INIT(0) };
 
 static DEFINE_SPINLOCK(pdrvdata_lock);
 static DECLARE_COMPLETION(early_console_added);
@@ -1244,7 +1245,7 @@ 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;
+	port->cons.vtermno = atomic_inc_return(&pdrvdata.next_vtermno);
 
 	port->cons.hvc = hvc_alloc(port->cons.vtermno, 0, &hv_ops, PAGE_SIZE);
 	if (IS_ERR(port->cons.hvc)) {
@@ -1255,7 +1256,6 @@ static int init_port_console(struct port *port)
 		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;
-- 
2.38.1