CVE-2021-39656

What it actually does

Use-after-free in __configfs_open_file of file.c due to improper locking, allowing local kernel memory corruption with system execution privileges.

Why it matters to the business

Attackers already holding high privileges on affected systems could gain full kernel control, risking complete device compromise and data exposure across Android-based deployments.

How an attack unfolds

An attacker with system execution privileges interacts with configfs files to trigger the locking flaw, causing a use-after-free that is leveraged to corrupt kernel structures and escalate privileges without user interaction.

How to fix it

Apply the kernel patch corresponding to upstream commit a3b3a9481091ddd06b8f83784ba9c4e034dc912a or upgrade affected Android kernel builds to a version containing the fix; verify the change is present in the running kernel.

How to confirm the fix worked

Confirm the fixed commit or patched kernel version is deployed, then attempt to reproduce the configfs file access pattern under load to verify the locking now prevents the use-after-free.

Typical attack path

1. Obtain system execution privileges on the target Android device
2. Manipulate configfs file operations to hit the improper locking path
3. Trigger the use-after-free condition in kernel memory
4. Leverage the resulting corruption for privilege escalation to kernel level

How to detect it

Review kernel logs and audit trails for unusual configfs file operations or oops/panic messages around file.c; correlate with process activity that opens configfs entries while holding elevated privileges.

What your security team should do

• Prioritize patching Android kernel devices within the next maintenance window given the local privilege escalation risk
• Audit systems for any custom configfs usage that could increase exposure surface
• Monitor for future Android security bulletins that reference similar kernel locking issues