CVE-2020-29660

What it actually does

The flaw permits a read-after-free condition in the Linux kernel tty subsystem when handling the TIOCGSID ioctl due to locking inconsistencies between tty_io.c and tty_jobctrl.c, allowing access to freed memory structures.

Why it matters to the business

On affected Linux systems this can result in limited local information disclosure by privileged users, increasing risk of credential or data leakage on servers and endpoints; the issue has been public for years with available exploits but no widespread ransomware use, lowering immediate operational urgency.

How an attack unfolds

An attacker with local root privileges triggers a race condition in tty session ID handling to perform a read on already-freed kernel memory associated with a controlling terminal, potentially disclosing sensitive kernel data.

How to fix it

Update the Linux kernel to a version containing commit c8bcd9c5be24fb9e6132e97da5a35e55a83e36b9 or later; rebuild and reboot into the patched kernel, then verify the tty subsystem no longer exhibits the locking flaw.

How to confirm the fix worked

Confirm the running kernel includes the referenced commit via git log or kernel version string; test TIOCGSID ioctl behavior under concurrent session changes to ensure no read-after-free occurs.

Typical attack path

1. Obtain local high-privilege access on a vulnerable Linux kernel
2. Open a controlling tty and issue TIOCGSID ioctl while manipulating session state
3. Trigger the locking inconsistency to dereference freed memory
4. Read and exfiltrate kernel memory contents for further attacks

How to detect it

Examine kernel audit logs, dmesg output, and system call traces for unusual TIOCGSID ioctl usage combined with tty session manipulation; look for crash reports or memory corruption indicators in kernel logs on systems running kernels before the fix commit.

What your security team should do

• Prioritize patching all Linux kernels to the fixed commit within standard maintenance windows
• Restrict local root access and tty device permissions where feasible
• Monitor for new kernel releases and integrate automated update testing
• Review exploit maturity reports and apply compensating controls if unpatched systems remain