On Thu, Feb 03, 2022 at 08:18:46AM -0800, Tadeusz Struk wrote: > 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. This results it a crash as shown below: > > > 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. > > Link: https://syzkaller.appspot.com/bug?id=9d9c27adc674e3a7932b22b61c79a02da82cbdc1 > Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group") > Reported-by: syzbot+af7a719bc92395ee41b3@xxxxxxxxxxxxxxxxxxxxxxxxx > Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx> > Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx> Thanks!