CVE-2025-40304

Vulnerability

CVE-2025-40304 is a memory safety vulnerability in the Linux kernel's framebuffer (fbdev) subsystem. The bit_putcs function, which renders text to the framebuffer, lacked proper bounds checking for text positioned near screen edges. When the Y position is off-screen, the function now returns early; when the X position is off-screen, it breaks from the rendering loop. Additionally, when clipping the image width to fit the screen, the character count (cnt) must be updated to match the clipped width. Without this update, bit_putcs_aligned and bit_putcs_unaligned receive mismatched parameters: the buffer is allocated for the clipped width, but cnt reflects the original larger count, leading to out-of-bounds writes past the allocated buffer [1][2][3].

Exploitation

An attacker would need the ability to trigger text rendering in the framebuffer console, which typically requires local access or the ability to influence console output (e.g., through a terminal or a crafted display operation). The vulnerability is triggered by rendering text at screen-edge coordinates, causing the rendering functions to write beyond the allocated framebuffer memory. No special privileges beyond the ability to write to the console are required, making it a low-complexity local attack.

Impact

Successful exploitation allows an attacker to write out-of-bounds to kernel memory allocated via vmalloc. vmalloc. This can corrupt adjacent kernel data structures, potentially leading to system crashes (denial of service) or, in more sophisticated scenarios, privilege escalation. The vulnerability is classified as a memory safety issue with high severity due to the potential for arbitrary write primitives.

Mitigation

The fix was committed to the Linux kernel stable tree in multiple commits [1][2][3]. Users should update their kernel to a version containing these patches. The vulnerability is not known to be exploited in the wild, but given its recent disclosure (December 2025). No workaround is available; patching is the recommended mitigation.