[PATCH v2] 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.

Fixes: 84a9582fd203 ("serial: core: Start managing serial controllers to enable runtime PM")
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=9f57c1b2792029198fcf
Cc: [email protected]
Assisted-by: Claude:claude-opus-5
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Ruslan Valiyev <[email protected]>
---
Reproduced on 8d3ae59288f1 with syzbot's config under QEMU/KVM x86_64:
6/6 runs oops on stock, 0/6 patched.  checkpatch clean, no new W=1
warnings.

The reproducer still does not run to completion on a patched kernel.  It
goes on to hit two pre-existing problems in the tty layer that this patch
does not touch: tty_cdev_add() leaves driver->cdevs[index] pointing at a
freed cdev when cdev_add() fails, and tty_unregister_device() deletes that
entry unconditionally when it is NULL.  Mentioning it so the remaining
crashes are not mistaken for this fix failing.

v1: https://lore.kernel.org/all/[email protected]/
v2: trimmed the commit message and backtrace per Andy Shevchenko's
    review, added the Assisted-by tags.
 drivers/tty/serial/serial_core.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index a530ad372b434..5bf71d7bbd223 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -3327,6 +3327,7 @@ 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);
@@ -3341,12 +3342,24 @@ 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);

base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f
-- 
2.43.0