sched_ext: Preserve rq tracking across local DSQ dispatch

In the Linux kernel, the following vulnerability has been resolved: sched_ext: Preserve rq tracking across local DSQ dispatch dispatch_to_local_dsq() can run from scx_bpf_dsq_move_to_local() while ops.dispatch() has recorded the current rq. Moving a task to a local DSQ may switch to the source or destination rq before synchronously invoking ops.dequeue() through the following path: SCX_CALL_OP(dispatch, rq) ops.dispatch() scx_bpf_dsq_move_to_local() scx_flush_dispatch_buf() finish_dispatch() dispatch_to_local_dsq() scx_dispatch_enqueue() local_dsq_post_enq() call_task_dequeue() SCX_CALL_OP_TASK(dequeue, locked_rq, ...) The nested callback saves the recorded rq and restores it on return. If the rq tracking does not follow the lock switch, update_locked_rq() can trigger the following lockdep assertion while restoring an rq which is no longer held: WARNING: kernel/sched/sched.h:1641 at call_task_dequeue+0x160/0x170 Call Trace: scx_dispatch_enqueue+0x2b0/0x460 dispatch_to_local_dsq+0x138/0x230 scx_flush_dispatch_buf+0x1af/0x220 scx_bpf_dsq_move_to_local___v2+0xe2/0x1c0 bpf__sched_ext_ops_dispatch+0x4b/0xa7 do_pick_task_scx+0x3b6/0x910 __pick_next_task+0x105/0x1f0 __schedule+0x3e7/0x1980 Introduce switch_rq_lock() to update the tracking state together with each rq lock handoff. Use it in dispatch_to_local_dsq(), move_remote_task_to_local_dsq() and the in-balance paths of scx_dsq_move(), ensuring that scx_locked_rq() consistently refers to the rq whose lock is actually held throughout the lock dance.

The current structured CVE record identifies a security weakness, but the root cause requires confirmation in the linked vendor material.

An attacker operating through the affected interface may attempt exploitation when the stated preconditions are met. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.