sched/fair: Fix fault in reweight_entity

Syzbot found a GPF in reweight_entity. This has been bisected to
commit 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid
sched_task_group")

There is a race between sched_post_fork() and setpriority(PRIO_PGRP)
within a thread group that causes a null-ptr-deref in
reweight_entity() in CFS. The scenario is that the main process spawns
number of new threads, which then call setpriority(PRIO_PGRP, 0, -20),
wait, and exit. For each of the new threads the copy_process() gets
invoked, which adds the new task_struct and calls sched_post_fork()
for it.

In the above scenario there is a possibility that
setpriority(PRIO_PGRP) and set_one_prio() will be called for a thread
in the group that is just being created by copy_process(), and for
which the sched_post_fork() has not been executed yet. This will
trigger a null pointer dereference in reweight_entity(), as it will
try to access the run queue pointer, which hasn't been set.

Before the mentioned change the cfs_rq pointer for the task has been
set in sched_fork(), which is called much earlier in copy_process(),
before the new task is added to the thread_group. Now it is done in
the sched_post_fork(), which is called after that. To fix the issue
the remove the update_load param from the update_load param() function
and call reweight_task() only if the task flag doesn't have the
TASK_NEW flag set.

Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
Reported-by: [e-mail]
Signed-off-by: Tadeusz Struk <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Dietmar Eggemann <[e-mail]>
Cc: [e-mail]
Link: https://lkml.kernel.org/r/[e-mail]

kernel/sched/core.c

···
 }
 #endif
 
-static void set_load_weight(struct task_struct *p, bool update_load)
+static void set_load_weight(struct task_struct *p)
 {
+	bool update_load = !(READ_ONCE(p->__state) & TASK_NEW);
 	int prio = p->static_prio - MAX_RT_PRIO;
 	struct load_weight *load = &p->se.load;
 
···
 			p->static_prio = NICE_TO_PRIO(0);
 
 		p->prio = p->normal_prio = p->static_prio;
-		set_load_weight(p, false);
+		set_load_weight(p);
 
 		/*
 		 * We don't need the reset flag anymore after the fork. It has
···
 		put_prev_task(rq, p);
 
 	p->static_prio = NICE_TO_PRIO(nice);
-	set_load_weight(p, true);
+	set_load_weight(p);
 	old_prio = p->prio;
 	p->prio = effective_prio(p);
 
···
 	 */
 	p->rt_priority = attr->sched_priority;
 	p->normal_prio = normal_prio(p);
-	set_load_weight(p, true);
+	set_load_weight(p);
 }
 
 /*
···
 #endif
 	}
 
-	set_load_weight(&init_task, false);
+	set_load_weight(&init_task);
 
 	/*
 	 * The boot idle thread does lazy MMU switching as well: