CVE-2024-35977: platform/chrome: cros_ec_uart: properly fix race condition

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

platform/chrome: crosecuart: properly fix race condition

The crosecuartprobe() function calls devmserdevdeviceopen() before it calls serdevdevicesetclientops(). This can trigger a NULL pointer dereference:

BUG: kernel NULL pointer dereference, address: 0000000000000000 ... Call Trace: <TASK> ... ? ttyportreceivebuf

A simplified version of crashing code is as follows:

static inline sizet serdevcontrollerreceivebuf(struct serdevcontroller ctrl, const u8 data, sizet count) { struct serdevdevice serdev = ctrl->serdev;

if (!serdev || !serdev->ops->receivebuf) // CRASH! return 0;

return serdev->ops->receivebuf(serdev, data, count); }

It assumes that if SERPORTACTIVE is set and serdev exists, serdev->ops will also exist. This conflicts with the existing crosecuartprobe() logic, as it first calls devmserdevdeviceopen() (which sets SERPORTACTIVE), and only later sets serdev->ops via serdevdevicesetclientops().

Commit 01f95d42b8f4 ("platform/chrome: crosecuart: 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 devmserdevdeviceopen().