CVE-2021-47524

What it actually does

The liteuart serial driver in the Linux kernel fails to release an allocated minor number when a device probe operation encounters an error and returns early.

Why it matters to the business

Repeated probe failures can exhaust the pool of available minor numbers, eventually preventing serial devices from being registered and disrupting terminal or console access on affected systems.

How an attack unfolds

An attacker with the ability to induce probe failures (for example via crafted hardware or module reload sequences) repeatedly triggers the error path, leaking minors until the allocator is exhausted and new serial devices can no longer be added.

How to fix it

Apply a kernel update containing the commit that releases the minor number before returning on probe errors; rebuild and reboot with the patched kernel.

How to confirm the fix worked

Confirm the presence of the fix by inspecting the liteuart_probe function for an ida_simple_remove or equivalent release call on the error path; verify via kernel version or source diff.

Typical attack path

1. Obtain local access or the ability to trigger liteuart probe operations
2. Force repeated probe failures through module unload/reload or hardware manipulation
3. Continue until the minor-number allocator returns errors for legitimate devices

How to detect it

Search kernel logs and dmesg for liteuart probe error messages that are not followed by minor-number release messages; monitor for ENOSPC or similar errors when additional serial devices attempt registration.

What your security team should do

• Prioritize kernel updates on systems that load the liteuart driver
• Review boot and dmesg logs for liteuart probe failures as an early indicator of exposure
• Test serial device functionality after patching to confirm minor numbers are reclaimed