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From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-arch@vger.kernel.org
Cc:     Tejun Heo <tj@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Al Viro <viro@ZenIV.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>,
        Linus Torvalds <torvalds@linuxfoundation.org>,
        linux-kernel@vger.kernel.org,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        stable@vger.kernel.org,
        =?utf-8?B?0JzQsNC60YHQuNC8INCa0YPRgtGP0LLQuNC9?= 
        <maximkabox13@gmail.com>
Subject: Re: [PATCH 1/7] kthread: Don't allocate kthread_struct for init and
 umh
In-Reply-To: <20220506141512.516114-1-ebiederm@xmission.com>
References: <87mtfu4up3.fsf@email.froward.int.ebiederm.org>
 <20220506141512.516114-1-ebiederm@xmission.com>
Date:   Tue, 10 May 2022 16:38:27 +0200
Message-ID: <87fslhpi58.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Fri, May 06 2022 at 09:15, Eric W. Biederman wrote:
>  	 * the init task will end up wanting to create kthreads, which, if
>  	 * we schedule it before we create kthreadd, will OOPS.
>  	 */
> -	pid = kernel_thread(kernel_init, NULL, CLONE_FS);
> +	pid = user_mode_thread(kernel_init, NULL, CLONE_FS);

So init does not have PF_KTHREAD set anymore, which causes this to go
sideways with a NULL pointer dereference in get_mm_counter() on next:

 get_mm_counter include/linux/mm.h:1996 [inline]
 get_mm_rss include/linux/mm.h:2049 [inline]
 task_nr_scan_windows.isra.0+0x23/0x120 kernel/sched/fair.c:1123
 task_scan_min kernel/sched/fair.c:1144 [inline]
 task_scan_start+0x6c/0x400 kernel/sched/fair.c:1150
 task_tick_numa kernel/sched/fair.c:2944 [inline]
 task_tick_fair+0xaeb/0xef0 kernel/sched/fair.c:11186
 scheduler_tick+0x20a/0x5e0 kernel/sched/core.c:5380

 https://lore.kernel.org/lkml/0000000000008a9fbb05dea76400@google.com

because the fence in task_tick_numa():

 	if ((curr->flags & (PF_EXITING | PF_KTHREAD)) || work->next != work)
		return;

is not longer sufficient. It needs also to bail if !curr->mm.

I'm worried that there are more of these issues lurking. Haven't looked
yet.

Thanks,

        tglx