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Subject[PATCH 5.16 040/186] sched/fair: Fix fault in reweight_entity
Date
From: Tadeusz Struk <tadeusz.struk@linaro.org>

[ 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
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: syzbot+af7a719bc92395ee41b3@syzkaller.appspotmail.com
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20220203161846.1160750-1-tadeusz.struk@linaro.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 d24823b3c3f9f..35b256b789680 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1203,8 +1203,9 @@ int tg_nop(struct task_group *tg, void *data)
}
#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;

@@ -4392,7 +4393,7 @@ 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
@@ -6879,7 +6880,7 @@ 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);

@@ -7170,7 +7171,7 @@ 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);
}

/*
@@ -9409,7 +9410,7 @@ void __init sched_init(void)
#endif
}

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

/*
* The boot idle thread does lazy MMU switching as well:
--
2.34.1


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