CVE-2021-47171

What it actually does

Unconditional memory leak of kzalloc-allocated structures in error paths after successful allocation inside smsc75xx_bind() when subsequent operations fail.

Why it matters to the business

Repeated attachment of affected USB Ethernet adapters under error conditions slowly exhausts kernel memory, eventually causing system instability, degraded performance, or crashes on Linux hosts using smsc75xx-based devices.

How an attack unfolds

An attacker with local access repeatedly triggers USB device probe paths that reach the error-handling branches after kzalloc in smsc75xx_bind, leaving allocated objects unfreed; no remote vector or authentication bypass is involved.

How to fix it

Upgrade to a Linux kernel containing the fix for the memory leak in smsc75xx_bind (post-2021 backports of the referenced commit); rebuild and reboot with the patched kernel; disable or avoid smsc75xx USB devices until updated.

How to confirm the fix worked

Confirm kernel source contains the error-path kfree or goto unwind added after the kzalloc in smsc75xx_bind; run syzkaller or USB attach stress tests that previously triggered the leak and verify no net increase in slab usage for the affected objects.

Typical attack path

1. Obtain local shell or physical USB access on target Linux system
2. Attach or emulate smsc75xx USB Ethernet device to invoke usbnet_probe
3. Force error conditions after the kzalloc at drivers/net/usb/smsc75xx.c:1460
4. Repeat attachment cycle to accumulate leaked kernel memory
5. Observe system-wide memory pressure or OOM conditions

How to detect it

Review dmesg and kernel logs for repeated smsc75xx_bind error paths; correlate USB device attach events with rising slab memory usage for the driver; inspect crash dumps for kmalloc objects still referenced only from the bind error path.

What your security team should do

• Prioritize kernel upgrade on all systems exposing USB networking with smsc75xx hardware
• Monitor slabtop or /proc/meminfo for sustained growth in kmalloc- objects tied to usbnet
• Restrict unprivileged USB device attachment via udev or physical port controls until patched
• Add the driver to automated regression test suites for future kernel updates