CVE-2021-47339

What it actually does

Failure to explicitly zero the ioctl input buffer before compat conversion allows uninitialized kernel stack bytes to reach driver ioctl handlers, enabling leakage of kernel memory contents to userspace.

Why it matters to the business

Attackers may obtain sensitive kernel memory contents, increasing the risk of credential or key exposure on systems that process untrusted video or media input.

How an attack unfolds

An attacker invokes a compat ioctl on a V4L2 device node with a crafted request that bypasses proper initialization; the kernel passes leftover stack data into a driver handler that then copies portions of that data back to userspace.

How to fix it

Upgrade to a Linux kernel containing the explicit buffer-clearing change in v4l2-ioctl.c; apply the stable patch series that zeros the entire ioctl buffer before calling compat handlers.

How to confirm the fix worked

Confirm the running kernel contains the commit that adds memset of the ioctl buffer before compat conversion; recompile or boot-test a kernel with the fix and verify no uninitialized data is returned on the same ioctl sequences.

Typical attack path

1. Open a V4L2 device node with write access
2. Issue a compat-mode ioctl with a request code known to exercise the vulnerable conversion path
3. Observe returned data containing kernel stack contents
4. Repeat or vary requests to map additional memory regions

How to detect it

Review audit logs or strace output for unusual V4L2 ioctl sequences returning unexpectedly large or patterned data; correlate with processes that should not receive kernel memory contents.

What your security team should do

• Prioritize kernel updates on any system exposing V4L2 devices to untrusted processes
• Restrict access to /dev/video* nodes via udev or device cgroup policies
• Monitor EPSS and upstream stable branches for related v4l2 fixes