Re: [PATCH v5] sched/fair: Fix fault in reweight_entity

[Peter Zijlstra]

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!