serial: Fix not set tty->port race condition

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

serial: Fix not set tty->port race condition

Revert commit bfc467db60b7 ("serial: remove redundant

tty_port_link_device()") because the tty_port_link_device() is not

redundant: the tty->port has to be confured before we call

uart_configure_port(), otherwise user-space can open console without TTY

linked to the driver.

This tty_port_link_device() was added explicitly to avoid this exact

issue in commit fb2b90014d78 ("tty: link tty and port before configuring

it as console"), so offending commit basically reverted the fix saying

it is redundant without addressing the actual race condition presented

there.

Reproducible always as tty->port warning on Qualcomm SoC with most of

devices disabled, so with very fast boot, and one serial device being

the console:

printk: legacy console [ttyMSM0] enabled

printk: legacy console [ttyMSM0] enabled

printk: legacy bootconsole [qcom_geni0] disabled

printk: legacy bootconsole [qcom_geni0] disabled

------------[ cut here ]------------

tty_init_dev: ttyMSM driver does not set tty->port. This would crash the kernel. Fix the driver!

WARNING: drivers/tty/tty_io.c:1414 at tty_init_dev.part.0+0x228/0x25c, CPU#2: systemd/1

Modules linked in: socinfo tcsrcc_eliza gcc_eliza sm3_ce fuse ipv6

CPU: 2 UID: 0 PID: 1 Comm: systemd Tainted: G S 6.19.0-rc4-next-20260108-00024-g2202f4d30aa8 #73 PREEMPT

Tainted: [S]=CPU_OUT_OF_SPEC

Hardware name: Qualcomm Technologies, Inc. Eliza (DT)

...

tty_init_dev.part.0 (drivers/tty/tty_io.c:1414 (discriminator 11)) (P)

tty_open (arch/arm64/include/asm/atomic_ll_sc.h:95 (discriminator 3) drivers/tty/tty_io.c:2073 (discriminator 3) drivers/tty/tty_io.c:2120 (discriminator 3))

chrdev_open (fs/char_dev.c:411)

do_dentry_open (fs/open.c:962)

vfs_open (fs/open.c:1094)

do_open (fs/namei.c:4634)

path_openat (fs/namei.c:4793)

do_filp_open (fs/namei.c:4820)

do_sys_openat2 (fs/open.c:1391 (discriminator 3))

...

Starting Network Name Resolution...

Apparently the flow with this small Yocto-based ramdisk user-space is:

driver (qcom_geni_serial.c): user-space:

============================ ===========

qcom_geni_serial_probe()

uart_add_one_port()

serial_core_register_port()

serial_core_add_one_port()

uart_configure_port()

register_console()

|

| open console

| ...

| tty_init_dev()

| driver->ports[idx] is NULL

|

tty_port_register_device_attr_serdev()

tty_port_link_device() <- set driver->ports[idx]