CVE-2026-98866 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
sched/cache: Introduce task_struct->sched_cache_grp to fix UAF
Add a sched_cache_grp pointer to task_struct so that scheduler code can access the cache group directly via the task, without going through mm->sched_cache_grp. This decouples the scheduler's hot-path accesses from the mm_struct.
Each task holds its own refcount on the sched_cache_group, separate from the reference held by its mm_struct. The reference is acquired in copy_mm() (fork) and exec_mmap() (exec), and released in exit_mm(). This fixes the use-after-free when account_mm_sched() reaches the group through a task whose mm is being switched, as reported by Hyunwoo:
https://lore.kernel.org/lkml/apPb-Dr4nPYuHQOK@v4bel/
Convert all scheduler code in fair.c and exit.c to use p->sched_cache_grp instead of p->mm->sched_cache_grp.
Keep the fork/exec/exit reference management out of the generic mm paths: add sched_cache_fork(), sched_cache_fork_cleanup(), sched_cache_exec_mmap() and sched_cache_exit_mm() in kernel/sched/cache_sched.c (with empty stubs for !CONFIG_SCHED_CACHE), so fs/exec.c, kernel/fork.c and kernel/exit.c each call one helper instead of open-coding the refcounting under #ifdef. Also add sched_cache_group_get() and task_cache_group_get().
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel scheduler subsystem contained a critical use-after-free vulnerability rooted in the management of reference counts for scheduling cache groups associated with memory descriptors. The flaw occurred because the scheduler's hot-path code accessed these cache groups through the mm_struct pointer embedded within each task structure. This indirect access created a race condition where, during context switches or process execution transitions, the mm_struct could be freed while the scheduler still held an active reference to it via the associated sched_cache_group. Specifically, when account_mm_sched attempted to reach the group through a task whose memory descriptor was in the process of being switched out or released, the underlying object might already have been deallocated by another thread, leading to undefined behavior and potential kernel crashes or privilege escalation vectors.
To resolve this architectural flaw, developers introduced a dedicated sched_cache_grp pointer directly within the task_struct structure. This change decouples the scheduler's frequent access patterns from the lifecycle of the mm_struct, ensuring that each task maintains its own independent reference count on the scheduling cache group. The reference acquisition logic was moved to specific points in the process lifecycle: copy_mm during fork operations and exec_mmap during execution updates acquire the reference, while exit_mm releases it upon process termination. This separation ensures that even if the mm_struct is freed or replaced, the scheduler retains a valid pointer to the necessary cache group data through the task itself, eliminating the window of vulnerability where stale pointers could be dereferenced.
The implementation involved converting all relevant scheduler code in fair.c and exit.c to utilize the new p->sched_cache_grp field instead of traversing through p->mm->sched_cache_grp. To maintain clean separation of concerns and avoid cluttering generic memory management paths with scheduling-specific logic, helper functions such as sched_cache_fork, sched_cache_exec_mmap, and sched_cache_exit_mm were added in kernel/sched/cache_sched.c. These helpers encapsulate the reference counting operations, allowing fs/exec.c, kernel/fork.c, and kernel/exit.c to call them directly without conditional compilation directives or open-coded refcounting logic. This modular approach enhances code maintainability while ensuring consistent handling of resource lifecycles across different subsystems.
From a security classification perspective, this vulnerability aligns with CWE-416: Use After Free, as the core issue involved accessing memory that had been freed due to improper reference management. In terms of attack vectors and detection methodologies, it relates to ATT&CK technique T1059: Command and Scripting Interpreter if exploited for arbitrary code execution via kernel panic or data corruption leading to system instability. The fix mitigates this by enforcing strict ownership semantics where the task structure owns its scheduling resources independently of memory descriptor changes.
System administrators should ensure that their Linux kernels are updated with patches addressing sched/cache improvements, particularly those involving task_struct modifications and reference counting for scheduler cache groups. Regular kernel updates are essential to prevent exploitation of such low-level race conditions. Additionally, monitoring system logs for unexpected oops or panic messages related to scheduling subsystems can help identify instances where the vulnerability might have been triggered before patching is applied. The structural changes also improve overall system stability by reducing contention and potential deadlocks associated with complex memory descriptor transitions during high-load scenarios.