CVE-2026-98219 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
sched_ext: Close the pre-enable ops error claim window
scx_alloc_and_add_sched() publishes ops->priv before scx_root_enable_workfn() switches the state to SCX_ENABLING. An error claimed via scx_bpf_error_bstr() from an associated BPF program in that window is consumed by scx_disable_workfn(), which takes the pre-enable shortcut in scx_root_disable(). The shortcut returns without any teardown and restores SCX_DISABLED with an unconditional scx_set_enable_state() xchg racing the enable workfn's own transition. The enable then completes with the claim consumed: the scheduler stays up but can never be disabled again, and bpf_scx_unreg() frees it while still in use, resulting in a use-after-free. Both WARN_ON_ONCE()s fire back to back:
WARNING: kernel/sched/ext/ext.c:7522 at scx_root_enable_workfn+0xeec/0x1be0, CPU#3: scx_enable_help/276
WARNING: kernel/sched/ext/ext.c:6398 at scx_root_disable+0xb50/0xdb8, CPU#0: sched_ext_helpe/664
scx_root_enable_workfn() switches to SCX_ENABLING before the scheduler allocation, so ops->priv is never visible while SCX_DISABLED. The allocation failure path restores SCX_DISABLED.
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel's scheduling extension framework contains a critical race condition within its enablement and disablement logic that can lead to use-after-free vulnerabilities and system instability. This flaw specifically affects the interaction between the scheduler core and associated BPF programs during the state transition from disabled to enabling states. The vulnerability arises because scx_alloc_and_add_sched() publishes the ops->priv pointer before the kernel switches the scheduler's internal state to SCX_ENABLING via scx_root_enable_workfn(). This timing gap creates a window where an error, claimed through scx_bpf_error_bstr from an associated BPF program, can be processed by scx_disable_workfn() while the system is in this intermediate state.
When such an error occurs within this specific time window, it triggers a pre-enable shortcut path inside scx_root_disable(). This shortcut attempts to restore the scheduler state to SCX_DISABLED using an unconditional atomic exchange operation via scx_set_enable_state(). However, because the enable work function has already initiated or is in the process of switching the state to SCX_ENABLING, this race condition results in conflicting state transitions. The disable path completes without performing necessary teardown procedures for the scheduler resources. Consequently, the scheduler remains technically active but enters an inconsistent state where it can never be properly disabled again.
The operational impact of this vulnerability is severe, leading directly to a use-after-free scenario. Since the enablement process eventually completes and consumes the error claim that triggered the premature disable attempt, the system believes the scheduler is fully enabled. However, when bpf_scx_unreg() attempts to unregister or free the BPF program associated with the scheduler, it frees memory structures that are still in active use by the kernel's scheduling subsystem. This misuse of freed memory can lead to arbitrary code execution, denial of service, or data corruption depending on how the kernel handles the corrupted state and pointers. The vulnerability is further evidenced by back-to-back warnings firing from scx_root_enable_workfn() and scx_root_disable(), indicating a clear conflict in concurrent access to scheduler resources.
To mitigate this risk, the fix involves restructuring the order of operations within the enablement process. Specifically, scx_root_enable_workfn() must switch the state to SCX_ENABLING before performing the scheduler allocation. This ensures that ops->priv is never visible or accessible while the system remains in the SCX_DISABLED state. By aligning the visibility of private data with the actual operational state, the race condition window is eliminated. The allocation failure path correctly restores the SCX_DISABLED state without exposing partially initialized structures to concurrent disable attempts. This change ensures that any error handling during enablement does not trigger premature teardown or inconsistent state transitions that could lead to resource leaks or use-after-free conditions.
From a vulnerability classification perspective, this issue aligns with CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, as the race condition stems from improper locking and ordering of shared resources during state changes. It also relates to CWE-416: Use After Free, due to the potential for accessing memory after it has been freed by unregister routines. In terms of ATT&CK mapping, this vulnerability could be leveraged in an Initial Access or Privilege Escalation context if exploited locally to crash the system or gain unauthorized access through kernel memory corruption. System administrators should apply the latest kernel updates that include this fix and monitor for any unusual scheduling behavior or kernel warnings related to sched_ext components.