CVE-2021-47173

What it actually does

The uss720_probe function allocates a usbdev structure via usb_alloc_dev but never calls usb_put_dev to decrement its reference count, causing a permanent memory leak of 2048-byte objects each time the probe path executes.

Why it matters to the business

Systems using the affected USB-to-serial adapter driver gradually exhaust kernel memory, leading to degraded performance, possible denial of service, or forced reboots on long-running Linux hosts that repeatedly connect the device.

How an attack unfolds

An attacker with local access connects a compatible USB device that triggers the uss720 driver probe; the missing refcount decrement leaks memory on every attachment event until the system runs out of resources.

How to fix it

Apply the upstream fix that adds usb_put_dev after the probe logic; update to a Linux kernel version containing commit that resolves the refcount leak in drivers/usb/misc/uss720.c.

How to confirm the fix worked

Confirm the presence of usb_put_dev call immediately after usb_get_dev or equivalent in uss720_probe; monitor /proc/meminfo or slabinfo for unreclaimed usbdev objects after device attach/detach cycles.

Typical attack path

1. Obtain local user or physical access to a Linux system
2. Attach a USB device handled by the uss720 driver
3. Trigger repeated probe events via device connect/disconnect cycles
4. Observe progressive kernel memory depletion until stability impact

How to detect it

Search kernel logs and slab allocator traces for repeated allocations from usb_alloc_dev without matching frees; inspect dmesg or /sys/kernel/debug/kmemleak reports for unreferenced usbdev objects originating from the uss720 probe path.

What your security team should do

• Prioritize kernel updates on systems exposing USB ports to untrusted devices
• Restrict physical or local USB access via policy and port-control mechanisms until patched
• Add kernel memory-leak monitoring to existing observability stack for early detection