Linux: tty: serial: 8250: protect against NULL uart->port.dev in register

In the Linux kernel, the following vulnerability has been resolved:

tty: serial: 8250: protect against NULL uart->port.dev in register

serial8250_register_8250_port() conditionally copies uart->port.dev from up->port.dev only when up->port.dev is non-NULL:

if (up->port.dev) { uart->port.dev = up->port.dev; ... }

So if both the existing uart slot and up have a NULL ->dev, uart->port.dev remains NULL. The very next ACPI companion check then dereferences it unconditionally:

if (!has_acpi_companion(uart->port.dev)) {

has_acpi_companion() reads dev->fwnode without a NULL guard (include/linux/acpi.h), so this NULL-derefs the kernel for the remaining no-dev case rather than just skipping the mctrl_gpio_init() initialisation as intended.

smatch flags the inconsistency:

drivers/tty/serial/8250/8250_core.c:767 serial8250_register_8250_port() error: 'uart->port.dev' could be null (see line 719)

Guard the call with a NULL check so register continues to work for callers that legitimately have no parent device (legacy non-OF/non-ACPI registrations).

No functional change for callers that pass a non-NULL ->dev.