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Subject[PATCH 5.15 038/262] sched/fair: Fix fault in reweight_entity
From: Tadeusz Struk <>

[ Upstream commit 13765de8148f71fa795e0a6607de37c49ea5915a ]

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

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")
Signed-off-by: Tadeusz Struk <>
Signed-off-by: Peter Zijlstra (Intel) <>
Reviewed-by: Dietmar Eggemann <>
Signed-off-by: Sasha Levin <>
kernel/sched/core.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c2dec6ce98091..57e5c79142964 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1199,8 +1199,9 @@ int tg_nop(struct task_group *tg, void *data)

-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;

@@ -4358,7 +4359,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
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
@@ -6902,7 +6903,7 @@ void set_user_nice(struct task_struct *p, long nice)
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);

@@ -7193,7 +7194,7 @@ static void __setscheduler_params(struct task_struct *p,
p->rt_priority = attr->sched_priority;
p->normal_prio = normal_prio(p);
- set_load_weight(p, true);
+ set_load_weight(p);

@@ -9431,7 +9432,7 @@ void __init sched_init(void)

- set_load_weight(&init_task, false);
+ set_load_weight(&init_task);

* The boot idle thread does lazy MMU switching as well:

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