[PATCH] sched_ext: idle: Recheck prev_cpu after narrowing allowed mask

[David Carlier <devnexen@gmail.com>]

scx_select_cpu_dfl() narrows @allowed to @cpus_allowed & @p->cpus_ptr
when the BPF caller supplies a @cpus_allowed that differs from
@p->cpus_ptr and @p doesn't have full affinity. However,
@is_prev_allowed was computed against the original (wider)
@cpus_allowed, so the prev_cpu fast paths could pick a @prev_cpu that
is in @cpus_allowed but not in @p->cpus_ptr, violating the intended
invariant that the returned CPU is always usable by @p. The kernel
masks this via the SCX_EV_SELECT_CPU_FALLBACK fallback, but the
behavior contradicts the documented contract.

Move the @is_prev_allowed evaluation past the narrowing block so it
tests against the final @allowed mask.

Fixes: ee9a4e92799d ("sched_ext: idle: Properly handle invalid prev_cpu during idle selection")
Assisted-by: Claude <noreply@anthropic.com>
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 kernel/sched/ext_idle.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index 41785f65bbb2..bbb845f36a0a 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -464,12 +464,6 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 
 	preempt_disable();
 
-	/*
-	 * Check whether @prev_cpu is still within the allowed set. If not,
-	 * we can still try selecting a nearby CPU.
-	 */
-	is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed);
-
 	/*
 	 * Determine the subset of CPUs usable by @p within @cpus_allowed.
 	 */
@@ -486,6 +480,12 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 		}
 	}
 
+	/*
+	 * Check whether @prev_cpu is still within the allowed set. If not,
+	 * we can still try selecting a nearby CPU.
+	 */
+	is_prev_allowed = cpumask_test_cpu(prev_cpu, allowed);
+
 	/*
 	 * This is necessary to protect llc_cpus.
 	 */
-- 
2.53.0