sched_ext: Refuse cross-task select_cpu_from_kfunc calls

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select_cpu_from_kfunc() skipped pi_lock for @p when called from
ops.select_cpu() or another rq-locked SCX op, assuming the held lock
protects @p. scx_bpf_select_cpu_dfl() / __scx_bpf_select_cpu_and() accept an
arbitrary KF_RCU task_struct, so a caller in e.g. ops.select_cpu(p1) or
ops.enqueue(p1) can pass some other p2 - the held pi_lock / rq lock is p1's,
not p2's - and reading p2->cpus_ptr / nr_cpus_allowed races with
set_cpus_allowed_ptr() and migrate_disable_switch() on another CPU.

Abort the scheduler on cross-task calls in both branches: for
ops.select_cpu() use scx_kf_arg_task_ok() to verify @p is the wake-up
task recorded in current->scx.kf_tasks[] by SCX_CALL_OP_TASK_RET();
for other rq-locked SCX ops compare task_rq(p) against scx_locked_rq().

v2: Switch the in_select_cpu cross-task check from direct_dispatch_task
    comparison to scx_kf_arg_task_ok(). The former spuriously rejects when
    ops.select_cpu() calls scx_bpf_dsq_insert() first, then calls
    scx_bpf_select_cpu_*() on the same task. (Andrea Righi)

Fixes: 0022b32850 ("sched_ext: Decouple kfunc unlocked-context check from kf_mask")
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andrea Righi <arighi@nvidia.com>

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Tejun Heo 2026-04-24 14:31:36 -10:00

parent c0e8ddc76d

commit ea7c716a24

1 changed files with 17 additions and 2 deletions

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kernel/sched/ext_idle.c

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