linux-kernel - Re: [PATCH] sched/fair: Fix fault in reweight

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Date:   Wed, 19 Jan 2022 07:43:23 -0800
From:   Tadeusz Struk <tadeusz.struk@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Zhang Qiao <zhangqiao22@...wei.com>, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Fix fault in reweight_entity

On 1/19/22 01:32, Peter Zijlstra wrote:
> On Tue, Jan 18, 2022 at 05:24:17PM -0800, Tadeusz Struk wrote:
>> Syzbot found a GPF in reweight_entity. This has been bisected to commit
>> c85c6fadbef0 ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
> That's a stable commit, the real commit is 4ef0c5c6b5ba1f38f0ea1cedad0cad722f00c14a

This is what syzbot bisected it to. I will reference the original commit in the
next version.

> 
>> Looks like after this change there is a time window, when
>> task_struct->se.cfs_rq can be NULL. This can be exploited to trigger
>> null-ptr-deref by calling setpriority on that task.
> Looks like isn't good enough, either there is, in which case you explain
> the window, or there isn't in which case what are we doing here?

There surely is something wrong, otherwise it wouldn't crash.
I will try to narrow down the reproducer to better understand what causes
the fault.

-- 
Thanks,
Tadeusz