Re: [PATCH 5.10 1/1] sched/fair: Fix fault in reweight_entity

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Aug 19, 2022 at 10:11:40AM +0300, Fedor Pchelkin wrote:
> 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' 

Now queued up, thanks.

greg k-h