CVE-2026-31733

Summary

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: Fix stale direct dispatch state in ddsp_dsq_id

@p->scx.ddsp_dsq_id can be left set (non-SCX_DSQ_INVALID) triggering a spurious warning in mark_direct_dispatch() when the next wakeup's ops.select_cpu() calls scx_bpf_dsq_insert(), such as:

WARNING: kernel/sched/ext.c:1273 at scx_dsq_insert_commit+0xcd/0x140

The root cause is that ddsp_dsq_id was only cleared in dispatch_enqueue(), which is not reached in all paths that consume or cancel a direct dispatch verdict.

Fix it by clearing it at the right places:

• direct_dispatch(): cache the direct dispatch state in local variables and clear it before dispatch_enqueue() on the synchronous path. For the deferred path, the direct dispatch state must remain set until process_ddsp_deferred_locals() consumes them.

• process_ddsp_deferred_locals(): cache the dispatch state in local variables and clear it before calling dispatch_to_local_dsq(), which may migrate the task to another rq.

• do_enqueue_task(): clear the dispatch state on the enqueue path (local/global/bypass fallbacks), where the direct dispatch verdict is ignored.

• dequeue_task_scx(): clear the dispatch state after dispatch_dequeue() to handle both the deferred dispatch cancellation and the holding_cpu race, covering all cases where a pending direct dispatch is cancelled.

• scx_disable_task(): clear the direct dispatch state when transitioning a task out of the current scheduler. Waking tasks may have had the direct dispatch state set by the outgoing scheduler's ops.select_cpu() and then been queued on a wake_list via ttwu_queue_wakelist(), when SCX_OPS_ALLOW_QUEUED_WAKEUP is set. Such tasks are not on the runqueue and are not iterated by scx_bypass(), so their direct dispatch state won't be cleared. Without this clear, any subsequent SCX scheduler that tries to direct dispatch the task will trigger the WARN_ON_ONCE() in mark_direct_dispatch().

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/ca685511f7afd42cdcbb0feea42e5d332d384251
• https://git.kernel.org/stable/c/5e7b2cc8fae9ec2a5bc53311191d2faaff75a4b5
• https://git.kernel.org/stable/c/7ea601daa0153e19cd1c6e6b300348c70c05fe77
• https://git.kernel.org/stable/c/7e0ffb72de8aa3b25989c2d980e81b829c577010

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