CVE-2026-80785 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
fbdev: serialize mode sysfs access with lock_fb_info()
show_mode(), show_modes(), and store_mode() access fb_info->modelist and fb_info->mode without holding lock_fb_info(). store_modes() takes lock_fb_info() while replacing the modelist and freeing the old one.
A concurrent reader or writer can load a pointer to an old modelist entry before store_modes() frees it, then dereference freed memory or store a stale freed pointer in fb_info->mode.
Take lock_fb_info() in show_mode(), show_modes(), and store_mode() to serialize with store_modes(). In show_mode(), copy the mode to the stack and format after dropping the lock. In store_mode(), split activate() into a _locked variant to avoid double-locking, and hold the locks for the modelist walk, mode conversion, activation, and fb_info->mode assignment together.
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Analysis
by VulDB Data Team • 09/04/2026
The Linux kernel framebuffer subsystem contains a concurrency vulnerability related to improper synchronization of access to display mode structures within the fb_info data structure. This issue specifically affects functions responsible for reading and writing video mode configurations via sysfs interfaces, namely show_mode, show_modes, and store_mode. These operations interact with critical fields such as modelist and mode pointers without acquiring the necessary serialization lock known as lock_fb_info. The absence of this lock creates a race condition where concurrent execution paths can interfere with each other during memory management and pointer updates.
The core technical flaw lies in the lack of mutual exclusion when accessing shared resources. While store_modes correctly acquires lock_fb_info while replacing the modelist and freeing old entries, the reader functions show_mode and show_modes do not hold this lock. This discrepancy allows a concurrent writer to free a memory block containing an old mode entry while a reader is still holding or about to dereference a pointer to that same block. Consequently, readers may access freed memory, leading to use-after-free conditions. Additionally, store_mode can write stale pointers into fb_info->mode if it does not properly serialize its operations with the writer function, potentially causing system instability or crashes when these invalid pointers are later utilized by other kernel components responsible for display output.
From a security perspective, this vulnerability is classified under CWE-416, Use After Free, as well as CWE-362, Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition). The attack vector involves local privilege escalation potential if an attacker can trigger these sysfs operations concurrently to exploit the race condition. By manipulating timing or triggering rapid changes in display modes, a malicious user could cause kernel memory corruption. This aligns with ATT&CK technique T1059, Command and Scripting Interpreter, as it may allow for arbitrary code execution through exploitation of the resulting crash or memory state inconsistency, although direct remote exploitation is not indicated here.
The operational impact includes potential system crashes leading to denial of service, data corruption within kernel memory spaces, and potentially privilege escalation if the use-after-free condition can be leveraged to overwrite function pointers or other critical control structures. The vulnerability affects systems relying on framebuffer devices for display output, particularly those where userspace applications frequently query or modify video modes through sysfs interfaces.
To mitigate this risk, the fix involves serializing all access to fb_info->modelist and fb_info->mode by acquiring lock_fb_info in show_mode, show_modes, and store_mode functions. For read operations like show_mode, the implementation copies the mode data to a local stack buffer while holding the lock and then formats the output after releasing it, ensuring no dangling pointers are accessed during formatting. For write operations like store_mode, the logic is refactored to hold locks throughout the entire sequence of modelist walking, mode conversion, activation, and assignment to fb_info->mode. This includes splitting activate functions into locked variants to prevent double-locking deadlocks while maintaining strict serialization with other concurrent writers. Administrators should apply kernel updates that include this patch immediately to restore proper synchronization mechanisms within the framebuffer subsystem.