sched: replace PF_THREAD_BOUND with PF_NO_SETAFFINITY

PF_THREAD_BOUND was originally used to mark kernel threads which were
bound to a specific CPU using kthread_bind() and a task with the flag
set allows cpus_allowed modifications only to itself.  Workqueue is
currently abusing it to prevent userland from meddling with
cpus_allowed of workqueue workers.

What we need is a flag to prevent userland from messing with
cpus_allowed of certain kernel tasks.  In kernel, anyone can
(incorrectly) squash the flag, and, for worker-type usages,
restricting cpus_allowed modification to the task itself doesn't
provide meaningful extra proection as other tasks can inject work
items to the task anyway.

This patch replaces PF_THREAD_BOUND with PF_NO_SETAFFINITY.
sched_setaffinity() checks the flag and return -EINVAL if set.
set_cpus_allowed_ptr() is no longer affected by the flag.

This will allow simplifying workqueue worker CPU affinity management.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 4 insertions, 5 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7f12624a393c..23606ee961b5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4126,6 +4126,10 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
         get_task_struct(p);
         rcu_read_unlock();
 
+        if (p->flags & PF_NO_SETAFFINITY) {
+                retval = -EINVAL;
+                goto out_put_task;
+        }
         if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL)) {
                 retval = -ENOMEM;
                 goto out_put_task;
@@ -4773,11 +4777,6 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
                 goto out;
         }
 
-        if (unlikely((p->flags & PF_THREAD_BOUND) && p != current)) {
-                ret = -EINVAL;
-                goto out;
-        }
-
         do_set_cpus_allowed(p, new_mask);
 
         /* Can the task run on the task's current CPU? If so, we're done */