From: Tadeusz Struk tadeusz.struk@linaro.org

commit 13765de8148f71fa795e0a6607de37c49ea5915a upstream.

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: Fedor Pchelkin pchelkin@ispras.ru --- 5.10 adaptation: Replaced a task_struct field '__state' with 'state'

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 e437d946b27b..86c5e08b393b 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -844,8 +844,9 @@ 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;

@@ -3262,7 +3263,7 @@ 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 @@ -5011,7 +5012,7 @@ 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);

@@ -5184,7 +5185,7 @@ 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); }

/* @@ -7189,7 +7190,7 @@ void __init sched_init(void) atomic_set(&rq->nr_iowait, 0); }

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

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