CVE-2023-38409

What it actually does

The flaw is a desynchronization between fbcon_registered_fb and fbcon_display arrays caused by set_con2fb_map only updating the mapping for the first virtual console; fbcon_mode_deleted then operates on stale fb_info pointers, enabling use-after-free memory corruption.

Why it matters to the business

Local attackers with console access can trigger kernel crashes or memory corruption on affected Linux systems, leading to denial of service; the issue has been public for over three years with no observed exploitation or ransomware use, and the fix has been available since mid-2023.

How an attack unfolds

An attacker with local access triggers framebuffer mode deletion after the initial mapping assignment, causing fbcon_mode_deleted to dereference a stale fb_info pointer from the desynchronized arrays and perform a use-after-free operation.

How to fix it

Upgrade the Linux kernel to a version containing commit e0ee50101346ca9cef52da75e3fb4380c27c042a or later; rebuild and reboot with the patched kernel.

How to confirm the fix worked

Confirm the running kernel contains commit e0ee50101346ca9cef52da75e3fb4380c27c042a by checking git log or kernel version strings; attempt to reproduce the desync condition in a test environment and verify absence of the stale pointer dereference.

Typical attack path

1. Obtain local user access on a vulnerable Linux kernel
2. Interact with the framebuffer console to cause set_con2fb_map to run for the first vc only
3. Trigger fbcon_mode_deleted while the arrays remain desynchronized
4. Cause the kernel to dereference the stale fb_info pointer resulting in memory corruption

How to detect it

Review kernel crash logs and dmesg output for fbcon-related oops or use-after-free warnings; search audit logs for unusual framebuffer or console mode changes by non-root processes.

What your security team should do

• Apply the kernel patch containing commit e0ee50101346ca9cef52da75e3fb4380c27c042a immediately on all affected systems
• Prioritize updates on systems exposing framebuffer or console interfaces to local users
• Monitor upstream Linux kernel releases for any related fbcon changes