platform/chrome: cros_ec_uart: properly fix race condition

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

platform/chrome: cros_ec_uart: properly fix race condition

The cros_ec_uart_probe() function calls devm_serdev_device_open() before

it calls serdev_device_set_client_ops(). This can trigger a NULL pointer

dereference:

BUG: kernel NULL pointer dereference, address: 0000000000000000

...

Call Trace:

<TASK>

...

? ttyport_receive_buf

A simplified version of crashing code is as follows:

static inline size_t serdev_controller_receive_buf(struct serdev_controller *ctrl,

const u8 *data,

size_t count)

{

struct serdev_device *serdev = ctrl->serdev;

if (!serdev || !serdev->ops->receive_buf) // CRASH!

return 0;

return serdev->ops->receive_buf(serdev, data, count);

}

It assumes that if SERPORT_ACTIVE is set and serdev exists, serdev->ops

will also exist. This conflicts with the existing cros_ec_uart_probe()

logic, as it first calls devm_serdev_device_open() (which sets

SERPORT_ACTIVE), and only later sets serdev->ops via

serdev_device_set_client_ops().

Commit 01f95d42b8f4 ("platform/chrome: cros_ec_uart: fix race

condition") attempted to fix a similar race condition, but while doing

so, made the window of error for this race condition to happen much

wider.

Attempt to fix the race condition again, making sure we fully setup

before calling devm_serdev_device_open().