A new security issue has been discovered and resolved in the Linux kernel's framebuffer (fbdev) subsystem. Labeled CVE-2025-40322, this bug allowed attackers to read memory outside the intended glyph array via faulty index calculation in bit_putcs_aligned() and bit_putcs_unaligned(). This vulnerability could let attackers leak sensitive information from memory, leading to possible data disclosure or system crashes.

Let's break down what happened, show you the affected code, how it can be exploited, and how it’s fixed—step by step, in plain language.

What is fbdev Bitblit?

The fbdev subsystem is a Linux kernel component that helps applications draw graphics on the screen. Bitblit functions are used to efficiently transfer pixel data (like drawing characters or images) inside the framebuffer using built-in font arrays.

The Vulnerability

Originally, the functions bit_putcs_aligned() and bit_putcs_unaligned() received a character (a Unicode codepoint), masked it to 8 or 9 bits (with xff or x1ff), and used the result as an index to look up a glyph in the built-in font array. But there was no check to ensure this index was within the actual number of glyphs.

This mistake could let the kernel read beyond the font array. Tools like syzbot (automated bug finder) caught this global out-of-bounds read.

Vulnerable Code Snippet

// VULNERABLE: index could be past font->glyh_num!
glyph = font->data + (ch & xff) * font->height;

If font->glyph_num is less than 256, (ch & xff) can be out of bounds!

How an Exploit Works

Attackers with access to framebuffer APIs could try to display specific characters that map the index beyond the font array, causing:

- Reading unrelated memory. The kernel could unintentionally show data from outside the font (potentially leaking sensitive info).

Simple Exploit Example (Pseudo-code)

// Send a request with a high character value to fbdev (e.g. via /dev/fb ioctl, DRM, or userland app)
write_glyph_to_screen(x200); // x200 > xff, may be out of glyph array bounds!

Repeat this with different values to scan memory near the font, looking for leaked data.

On some systems, you could directly echo data to /dev/fb mapped to a vulnerable driver

# This may differ for your real hardware, and permissions are needed!
echo -e "\x200" > /dev/fb

The Fix: Clamp to glyph count

The fixed code now ensures the index is never higher than font->glyph_num - 1.

Patched Code

// SAFE: clamp index
unsigned int glyph_index = ch & mask;
if (glyph_index >= font->glyph_num)
    glyph_index = ; // or some safe glyph
glyph = font->data + glyph_index * font->height;

By capping glyph_index at the available number of glyphs, the read stays in-bounds.

References & Original Commit

- Original Patch Commit (kernel.org)
- Syzbot Bug Report _(link for example, may need updating to your bug)_
- LKML Discussion Thread _(adjust as needed)_

Who is affected?

- Any Linux system using the fbdev driver with built-in fonts (common on embedded, legacy desktop, or virtual graphics hardware).

If you maintain your own kernel, backport the patch immediately.

- Limit or monitor access to /dev/fb and similar device files.

Conclusion

CVE-2025-40322 reminds us how small oversights in bounds checking can have big consequences in kernel code. Whenever you use an index from potentially unsafe data—even masked—always verify it stays within the expected array.

Timeline

Published on: 12/08/2025 01:16:04 UTC
Last modified on: 12/08/2025 18:26:19 UTC