[PATCH] sched/deadline: Fix spurious -EPERM for migration-disabled tasks

[Andrea Parri <parri.andrea@gmail.com>]

sched_setattr() can refuse to make a task SCHED_DEADLINE with -EPERM,
even though the task's affinity covers the whole root domain. A retry
can succeed, but the error blames an affinity the task does not have.

__sched_setscheduler() tests p->cpus_ptr, which is not always the task's
affinity. When a task is scheduled out inside a migrate_disable() section,
migrate_disable_switch() points p->cpus_ptr at the current CPU until
migrate_enable(). On PREEMPT_RT every spinlock_t holder is in such a
section, so ordinary kernel work opens the window. Elsewhere, seccomp
filters run in one.

Test p->cpus_mask instead, which keeps the task's affinity and is what
sched_getaffinity() reports. This should be safe because an admitted
task's cpus_mask covers the root domain, so set_cpus_allowed_dl() moves
no bandwidth when migrate_enable() restores p->cpus_ptr.

The following program, run as root, had 553679, 453720 and 424623 of its
1000000 requests refused on three boots of a 4-CPU PREEMPT_RT guest, and
236182, 217174 and 237183 without PREEMPT_RT. With this applied the count
is 0, also with lockdep, and a task pinned to one CPU is still refused:

  #define _GNU_SOURCE
  #include <errno.h>
  #include <linux/filter.h>
  #include <linux/seccomp.h>
  #include <pthread.h>
  #include <stddef.h>
  #include <stdint.h>
  #include <stdio.h>
  #include <sys/syscall.h>
  #include <unistd.h>

  #define REQUESTS 1000000
  #define INSNS 300

  struct attr {
      uint32_t size, policy;
      uint64_t flags;
      int32_t nice;
      uint32_t prio;
      uint64_t runtime, deadline, period;
  };

  static volatile int stop;
  static volatile pid_t target;

  static long set(struct attr a)
  {
      a.size = sizeof(a);
      return syscall(SYS_sched_setattr, target, &a, 0);
  }

  static void *worker(void *arg)
  {
      struct sock_filter f[INSNS + 1];
      struct sock_fprog prog = { .len = INSNS + 1, .filter = f };
      int i;

      /* Reading the arguments keeps seccomp from caching the result */
      for (i = 0; i < INSNS; i++)
          f[i] = (struct sock_filter)
              BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                       offsetof(struct seccomp_data, args));
      f[INSNS] = (struct sock_filter)
          BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW);
      syscall(SYS_seccomp, SECCOMP_SET_MODE_FILTER, 0, &prog);

      target = syscall(SYS_gettid);
      while (!stop)
          syscall(SYS_getppid);
      return arg;
  }

  static void *hog(void *arg)
  {
      while (!stop)
          ;
      return arg;
  }

  int main(void)
  {
      pthread_t t[9];
      long refused = 0;
      int i;

      pthread_create(&t[0], NULL, worker, NULL);
      while (!target)
          usleep(100);
      for (i = 1; i < 9; i++)
          pthread_create(&t[i], NULL, hog, NULL);
      for (i = 0; i < REQUESTS; i++) {
          /* SCHED_DEADLINE, 2us every 1s */
          if (set((struct attr){ .policy = 6, .runtime = 2000,
                                 .deadline = 1000000000,
                                 .period = 1000000000 }) && errno == EPERM)
              refused++;
          set((struct attr){ .policy = 0 }); /* SCHED_OTHER */
      }
      stop = 1;
      for (i = 0; i < 9; i++)
          pthread_join(t[i], NULL);
      printf("%ld of %d requests refused\n", refused, REQUESTS);
      return 0;
  }

An LLM helped find this bug and write this patch and its test.

Fixes: af449901b84c ("sched: Add migrate_disable()")
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Cc: stable@vger.kernel.org
---
This textually conflicts with [1], which rewrites this line and keeps
p->cpus_ptr. Whichever goes second needs to keep &p->cpus_mask.

Also tested on PREEMPT_RT with lockdep by holding a 1ms/10ms reservation
for 3ms at a time, so that tasks admitted while migration-disabled reach
migrate_enable() as SCHED_DEADLINE: no warnings, total_bw unchanged, and
none of 6788 requests refused. Without this patch, 826 of 6645 are refused.

Other !PREEMPT_RT migrate_disable() users were not tested.

[1] https://lore.kernel.org/20261030151307.768523-2-atomlin@atomlin.com/

 kernel/sched/syscalls.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 45a127deeadd9..fb8f332074b88 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -628,7 +628,7 @@ int __sched_setscheduler(struct task_struct *p,
 			 * the entire root_domain to become SCHED_DEADLINE. We
 			 * will also fail if there's no bandwidth available.
 			 */
-			if (!cpumask_subset(span, p->cpus_ptr) ||
+			if (!cpumask_subset(span, &p->cpus_mask) ||
 			    rq->rd->dl_bw.bw == 0) {
 				retval = -EPERM;
 				goto unlock;
-- 
2.53.0