CVE-2025-38685 in Linux
Summary
by MITRE • 09/04/2025
In the Linux kernel, the following vulnerability has been resolved:
fbdev: Fix vmalloc out-of-bounds write in fast_imageblit
This issue triggers when a userspace program does an ioctl FBIOPUT_CON2FBMAP by passing console number and frame buffer number. Ideally this maps console to frame buffer and updates the screen if console is visible.
As part of mapping it has to do resize of console according to frame buffer info. if this resize fails and returns from vc_do_resize() and continues further. At this point console and new frame buffer are mapped and sets display vars. Despite failure still it continue to proceed updating the screen at later stages where vc_data is related to previous frame buffer and frame buffer info and display vars are mapped to new frame buffer and eventully leading to out-of-bounds write in fast_imageblit(). This bheviour is excepted only when fg_console is equal to requested console which is a visible console and updates screen with invalid struct references in fbcon_putcs().
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Analysis
by VulDB Data Team • 02/10/2026
The vulnerability described in CVE-2025-38685 resides within the Linux kernel's framebuffer device implementation, specifically affecting the fbdev subsystem's handling of console-to-framebuffer mapping operations. This issue manifests through the FBIOPUT_CON2FBMAP ioctl command which allows userspace applications to map a console to a specific frame buffer device. The flaw occurs during the console resizing process when vc_do_resize() function fails to properly handle error conditions, leading to a cascade of improper state management that ultimately results in memory corruption.
The technical execution of this vulnerability begins with a userspace program issuing the FBIOPUT_CON2FBMAP ioctl call containing console and frame buffer identifiers. The kernel processes this request by attempting to map the console to the specified frame buffer and update screen display if the console is visible. During this mapping process, the system must resize the console according to the new frame buffer's specifications through the vc_do_resize() function. When this resize operation fails, the function returns early but the mapping process continues without proper error handling, creating a state where console and frame buffer are partially mapped while their internal data structures remain inconsistent.
The operational impact of this vulnerability stems from the improper memory management that occurs after the failed resize operation. Although the mapping process continues despite the resize failure, the system maintains references to the original frame buffer data structures while attempting to update display information using the new frame buffer configuration. This inconsistency creates a scenario where the fast_imageblit function attempts to write beyond the bounds of allocated memory regions, resulting in a potential out-of-bounds write condition. The vulnerability specifically manifests when the foreground console matches the requested console number, as this triggers the screen update process with invalid struct references in fbcon_putcs() function.
This vulnerability aligns with CWE-121 and CWE-787 categories, representing heap-based buffer overflow conditions that occur due to improper bounds checking in memory operations. The flaw demonstrates characteristics consistent with the ATT&CK technique T1068, which involves privilege escalation through exploitation of kernel vulnerabilities, as this issue could potentially be leveraged to execute arbitrary code with kernel privileges. The vulnerability affects the Linux kernel's graphics subsystem and represents a critical security flaw that could enable attackers to corrupt kernel memory and potentially gain system control. The issue requires careful handling of error conditions in kernel space operations and proper state management during console and frame buffer mapping processes.
Mitigation strategies for CVE-2025-38685 should include immediate kernel updates from vendors that contain the specific fix for the vmalloc out-of-bounds write condition in fast_imageblit. System administrators should implement monitoring for suspicious ioctl operations related to frame buffer mapping and console management. The fix addresses the root cause by ensuring proper error handling in vc_do_resize() function and preventing continuation of the mapping process when resize operations fail. Additionally, implementing proper bounds checking and memory validation in the framebuffer subsystem can help prevent similar issues in future implementations. Organizations should also consider restricting userspace access to framebuffer ioctl operations where possible and implement comprehensive kernel security measures including kernel address space layout randomization and control flow integrity protections.