[PATCH 4/4] sched/deadline: Reject SCHED_DEADLINE on exited tasks

[Juri Lelli]

sched_setattr(SCHED_DEADLINE) on a zombie task succeeds and charges
total_bw via sched_dl_overflow(), but the task has already done its
final schedule() and will never run again. Nothing releases the reserved
bandwidth. Repeating this fills total_bw to the admission cap, after
which sched_setattr(SCHED_DEADLINE) and CPU offline fail with -EBUSY.

Reject sched_setattr(SCHED_DEADLINE) on exited tasks with -ESRCH.

Reported-by: Weiheng Qiu <weihengqiuu@xxxxxxxxx>
Closes: https://lore.kernel.org/all/3f6e6c87-645b-45fd-8fc6-8da5bc5b82d8@xxxxxxxxx/
Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
---
 kernel/sched/syscalls.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 014697d454525..af9ba357a2956 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -583,6 +583,16 @@ int __sched_setscheduler(struct task_struct *p,
 	if (retval)
 		goto unlock;
 
+	/*
+	 * Reject SCHED_DEADLINE on exited tasks. A zombie will never be
+	 * scheduled again, so sched_dl_overflow() would charge total_bw
+	 * that nothing will ever release.
+	 */
+	if (dl_policy(policy) && p->exit_state) {
+		retval = -ESRCH;
+		goto unlock;
+	}
+
 	/*
 	 * If not changing anything there's no need to proceed further,
 	 * but store a possible modification of reset_on_fork.

-- 
2.56.0