[PATCH] sched, cpuset: Fix dl_cpu_busy() panic due to empty cs->cpus_allowed

[Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

With cgroup v2, the cpuset's cpus_allowed mask can be empty indicating
that the cpuset will just use the effective cpus of its parent. So
cpuset_can_attach() can call task_can_attach() with an empty mask.
This can lead to cpumask_any_and() returns nr_cpu_ids causing the call
to dl_bw_of() to crash due to percpu value access of an out of bound
cpu value. For example,

[80468.182258] BUG: unable to handle page fault for address: ffffffff8b6648b0
  :
[80468.191019] RIP: 0010:dl_cpu_busy+0x30/0x2b0
  :
[80468.207946] Call Trace:
[80468.208947]  cpuset_can_attach+0xa0/0x140
[80468.209953]  cgroup_migrate_execute+0x8c/0x490
[80468.210931]  cgroup_update_dfl_csses+0x254/0x270
[80468.211898]  cgroup_subtree_control_write+0x322/0x400
[80468.212854]  kernfs_fop_write_iter+0x11c/0x1b0
[80468.213777]  new_sync_write+0x11f/0x1b0
[80468.214689]  vfs_write+0x1eb/0x280
[80468.215592]  ksys_write+0x5f/0xe0
[80468.216463]  do_syscall_64+0x5c/0x80
[80468.224287]  entry_SYSCALL_64_after_hwframe+0x44/0xae

Fix that by using effective_cpus instead. For cgroup v1, effective_cpus
is the same as cpus_allowed. For v2, effective_cpus is the real cpumask
to be used by tasks within the cpuset anyway.

Also update task_can_attach()'s 2nd argument name to cs_effective_cpus to
reflect the change. In addition, a check is added to task_can_attach()
to guard against the possibility that cpumask_any_and() may return a
value >= nr_cpu_ids.

Fixes: 7f51412a415d ("sched/deadline: Fix bandwidth check/update when migrating tasks between exclusive cpusets")
Signed-off-by: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---
 include/linux/sched.h  | 2 +-
 kernel/cgroup/cpuset.c | 2 +-
 kernel/sched/core.c    | 8 +++++---
 3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 88b8817b827d..6a060160f0db 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1813,7 +1813,7 @@ current_restore_flags(unsigned long orig_flags, unsigned long flags)
 }
 
 extern int cpuset_cpumask_can_shrink(const struct cpumask *cur, const struct cpumask *trial);
-extern int task_can_attach(struct task_struct *p, const struct cpumask *cs_cpus_allowed);
+extern int task_can_attach(struct task_struct *p, const struct cpumask *cs_effective_cpus);
 #ifdef CONFIG_SMP
 extern void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask);
 extern int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask);
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 71a418858a5e..58aadfda9b8b 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2239,7 +2239,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 		goto out_unlock;
 
 	cgroup_taskset_for_each(task, css, tset) {
-		ret = task_can_attach(task, cs->cpus_allowed);
+		ret = task_can_attach(task, cs->effective_cpus);
 		if (ret)
 			goto out_unlock;
 		ret = security_task_setscheduler(task);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5555e49c4e12..addc3c2d2122 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8980,7 +8980,7 @@ int cpuset_cpumask_can_shrink(const struct cpumask *cur,
 }
 
 int task_can_attach(struct task_struct *p,
-		    const struct cpumask *cs_cpus_allowed)
+		    const struct cpumask *cs_effective_cpus)
 {
 	int ret = 0;
 
@@ -8999,9 +8999,11 @@ int task_can_attach(struct task_struct *p,
 	}
 
 	if (dl_task(p) && !cpumask_intersects(task_rq(p)->rd->span,
-					      cs_cpus_allowed)) {
-		int cpu = cpumask_any_and(cpu_active_mask, cs_cpus_allowed);
+					      cs_effective_cpus)) {
+		int cpu = cpumask_any_and(cpu_active_mask, cs_effective_cpus);
 
+		if (unlikely(cpu >= nr_cpu_ids))
+			return -EINVAL;
 		ret = dl_cpu_busy(cpu, p);
 	}
 
-- 
2.31.1