FAILED: patch "[PATCH] serial: core: fix NULL pointer dereference in" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 499485a67fbacf4d9afbdb43522388c64f853428
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026100502-chute-cranial-b5eb@gregkh' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 499485a67fbacf4d9afbdb43522388c64f853428 Mon Sep 17 00:00:00 2001
From: Ruslan Valiyev <[email protected]>
Date: Wed, 26 Aug 2026 10:46:54 +0200
Subject: [PATCH] serial: core: fix NULL pointer dereference in
serial_core_unregister_port()

port->port_dev is NULL when no port device is installed: it is cleared on teardown, and never set if registration failed before serial_core_port_device_add(). serial_core_unregister_port() passes it straight to serial_core_get_ctrl_dev(), which dereferences it:

  KASAN: null-ptr-deref in range [0x0000000000000040-0x0000000000000047]
  RIP: serial_core_unregister_port
  Call Trace:
   serial8250_unregister_port
   serial8250_remove
   unbind_store

Return early when there is no port device, and read port->port_dev under port_mutex.

Also clear port->port_dev on the serial_core_register_port() error path, where the port device has already been removed.

index 33c8f3b6ac81..6332ed545c89 100644
@@ -3381,6 +3381,7 @@ int serial_core_register_port(struct uart_driver *drv, struct uart_port *port)
 
 err_unregister_port_dev:
 	serial_base_port_device_remove(port->port_dev);
+	port->port_dev = NULL;
 
 err_unregister_ctrl_dev:
 	serial_base_ctrl_device_remove(new_ctrl_dev);

@@ -3395,12 +3396,24 @@ int serial_core_register_port(struct uart_driver *drv, struct uart_port *port)
 void serial_core_unregister_port(struct uart_driver *drv, struct uart_port *port)
 {
 	struct device *phys_dev = port->dev;
-	struct serial_port_device *port_dev = port->port_dev;
-	struct serial_ctrl_device *ctrl_dev = serial_core_get_ctrl_dev(port_dev);
+	struct serial_port_device *port_dev;
+	struct serial_ctrl_device *ctrl_dev;
 	int ctrl_id = port->ctrl_id;
 
 	guard(mutex)(&port_mutex);
 
+	/*
+	 * A NULL port device means there is no registered port device to
+	 * remove: serial_core_remove_one_port() clears port_dev on
+	 * teardown, and it is never set if registration failed before
+	 * serial_core_port_device_add().
+	 */
+	port_dev = port->port_dev;
+	if (!port_dev)
+		return;
+
+	ctrl_dev = serial_core_get_ctrl_dev(port_dev);
+
 	port->flags |= UPF_DEAD;
 
 	serial_core_remove_one_port(drv, port);