CVE-2026-63826 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
fbdev: fix use-after-free in store_modes()
store_modes() replaces a framebuffer's modelist with modes from userspace. On success it frees the old modelist with fb_destroy_modelist(). Two fields still point into that freed list.
One pointer is fb_display[i].mode, the mode a console is using.
fbcon_new_modelist() moves these pointers to the new list. It only does so for consoles still mapped to the framebuffer. An unmapped console is skipped and keeps its stale pointer. Unbinding fbcon, for example, sets con2fb_map[i] to -1 but leaves fb_display[i].mode set. An
FBIOPUT_VSCREENINFO ioctl with FB_ACTIVATE_INV_MODE later reaches fbcon_mode_deleted(). That function reads the stale fb_display[i].mode
through fb_mode_is_equal(). The read is a use-after-free.
The other pointer is fb_info->mode, the current mode. It is set through the mode sysfs attribute. store_modes() does not update fb_info->mode, so it is left pointing into the freed list. show_mode(), the attribute's read handler, dereferences the stale fb_info->mode through mode_string(). The read is a use-after-free.
Clear both pointers before freeing the list. Commit a1f305893074 ("fbcon: Set fb_display[i]->mode to NULL when the mode is released") added the
helper fbcon_delete_modelist(). It clears every fb_display[i].mode that
points into a given list. So far it is called only from the unregister path. Call it from store_modes() too, and set fb_info->mode to NULL.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability described represents a critical use-after-free condition within the Linux framebuffer subsystem that affects the fbdev driver implementation. This flaw exists in the store_modes() function which handles the replacement of framebuffer modelists with user-space provided modes. The core issue arises from improper memory management during the transition between old and new modelist structures, creating persistent dangling pointers that can be dereferenced long after the original memory has been freed.
The technical implementation involves two distinct but related use-after-free scenarios within the framebuffer console subsystem. The first occurs with fb_display[i].mode pointers that reference modes within the freed modelist structure. When fbcon_new_modelist() processes console mode updates, it only transfers pointers for consoles that remain mapped to the framebuffer, leaving unmapped consoles with stale references. This situation is exacerbated when fbcon unbinding operations set con2fb_map[i] to -1 while leaving fb_display[i].mode pointing to deallocated memory. Subsequent FBIOPUT_VSCREENINFO ioctls with FB_ACTIVATE_INV_MODE trigger fbcon_mode_deleted() which reads these stale pointers through fb_mode_is_equal(), resulting in the use-after-free condition.
The second vulnerability involves fb_info->mode pointer that maintains a reference to the current mode within the freed modelist. This pointer is not updated during store_modes() execution, leaving it pointing to deallocated memory structures. When the mode sysfs attribute's show_mode() handler attempts to read this stale reference through mode_string(), another use-after-free occurs. Both of these scenarios directly violate fundamental memory safety principles and can lead to arbitrary code execution or system crashes.
This vulnerability maps to CWE-416, specifically use-after-free conditions, and aligns with ATT&CK technique T1059 for privilege escalation through kernel exploits. The flaw affects the Linux kernel's framebuffer console subsystem and represents a classic memory management error where deallocated memory is not properly cleared before being referenced by subsequent operations. The fix implementation requires clearing both pointer references before freeing the modelist, specifically calling the existing helper function fbcon_delete_modelist() from store_modes() and setting fb_info->mode to NULL. This approach ensures proper memory cleanup and prevents dangling pointer dereferences that could be exploited by malicious actors to gain unauthorized access or cause system instability. The vulnerability demonstrates how seemingly simple memory management operations can create complex security issues when not properly synchronized with all referencing pointers throughout the system.