CVE-2026-80579 in Linux
Summary
by MITRE • 08/26/2026
In the Linux kernel, the following vulnerability has been resolved:
fbdev: clear fb_info->mode before deleting a videomode
fb_set_var() can delete a mode from info->modelist when userspace passes FB_ACTIVATE_INV_MODE through FBIOPUT_VSCREENINFO. The code checks that the mode being deleted is not the current info->var and that fbcon is not using it, but it does not check fb_info->mode.
fb_info->mode may still point into the modelist entry being deleted. If the entry is freed, later mode sysfs reads through show_mode() can dereference a stale pointer.
Clear fb_info->mode before calling fb_delete_videomode() when it matches the mode being removed.
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Analysis
by VulDB Data Team • 08/26/2026
The vulnerability identified in the Linux kernel's framebuffer subsystem represents a classic use-after-free condition arising from improper state management during dynamic video mode configuration changes. The core of this issue lies within the fb_set_var function, which is responsible for handling requests to modify display parameters sent by user-space applications via the FBIOPUT_VSCREENINFO ioctl interface. When an application passes the FB_ACTIVATE_INV_MODE flag, it signals a request to delete or invalidate a specific video mode from the kernel's internal modelist structure. While the existing code logic correctly verifies that the targeted mode is not currently active in fb_info->var and ensures that the framebuffer console (fbcon) is not actively rendering using that mode, it fails to perform an equally critical check on the fb_info->mode pointer. This oversight creates a scenario where the kernel retains a reference to a memory structure that is subsequently deallocated, leading to potential system instability or security exploitation.
From a technical perspective, this flaw constitutes a dangling pointer vulnerability within the device driver layer of the operating system. The fb_info structure serves as the central data container for framebuffer devices, and its mode member typically points to one of the entries in the modelist list that defines available display resolutions and refresh rates. When fb_delete_videomode is invoked to remove an entry from this list, it frees the associated memory resources. However, because the code does not explicitly nullify or update fb_info->mode when it references the same structure being deleted, the pointer remains valid in terms of its address but invalid in terms of content integrity. This stale reference persists until a subsequent operation attempts to access it, creating a window of opportunity for exploitation if an attacker can control the timing and sequence of memory allocations that might reuse the freed block or trigger a read before reallocation occurs.
The operational impact of this vulnerability is primarily observed through system crashes or undefined behavior when user-space applications attempt to query display mode information via sysfs interfaces. Specifically, functions like show_mode() are designed to expose video mode details to userspace for diagnostic and configuration purposes. If these functions execute after the underlying videomode structure has been freed but before fb_info->mode is cleared, they will dereference a stale pointer. This typically results in an invalid memory access error, which can manifest as a kernel panic or oops, effectively causing a denial of service against the system. In more sophisticated attack scenarios involving heap spraying techniques, such use-after-free conditions could potentially be leveraged to achieve arbitrary code execution by controlling the contents of the freed memory region before it is accessed again, although the primary and most immediate risk remains system stability.
This vulnerability aligns with CWE-416, which describes Use After Free errors where a pointer continues to point at an object after that object has been deallocated. Furthermore, from a tactical perspective within the MITRE ATT&CK framework for Linux systems, this flaw relates to techniques involving exploitation of resource management vulnerabilities, specifically those that allow attackers to disrupt service availability or potentially escalate privileges through memory corruption. The failure to maintain pointer consistency during state transitions is a common pattern in low-level system programming and highlights the necessity for rigorous validation of all references when modifying shared data structures.
To mitigate this vulnerability, developers must ensure that any pointers within kernel data structures are explicitly cleared or updated before the associated memory is freed. In this specific case, the fix involves checking if fb_info->mode points to the mode entry being deleted and setting it to NULL prior to calling fb_delete_videomode(). This ensures that subsequent reads through sysfs will detect a null pointer rather than dereferencing invalid memory, thereby preventing crashes and maintaining system integrity. For administrators managing systems with affected kernel versions, applying the latest security patches provided by distribution vendors is essential. Additionally, monitoring for unusual kernel log messages related to framebuffer operations or unexpected reboots can help in identifying potential exploitation attempts while patching strategies are implemented across the infrastructure.