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Date: Fri, 5 May 2017 18:44:28 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Vegard Nossum <vegard.nossum@oracle.com>
Cc: linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Jamie Iles <jamie.iles@oracle.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH] kthread: fix use-after-free if kthread fork fails
Message-ID: <20170505164428.GA500@redhat.com>
References: <20170505162034.4338-1-vegard.nossum@oracle.com>
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On 05/05, Vegard Nossum wrote:
>
> If a kthread forks (e.g. usermodehelper since commit 1da5c46fa965) but
> fails in copy_process() between calling dup_task_struct() and setting
> p->set_child_tid, then the value of p->set_child_tid will be inherited
> from the parent and get prematurely freed by free_kthread_struct().

Aaah... thanks!

> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -518,6 +518,13 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
>  	atomic_set(&tsk->stack_refcount, 1);
>  #endif
>  
> +	/*
> +	 * Forking kthreads (e.g. usermodehelper) should not inherit this
> +	 * field since it's a pointer to a 'struct kthread' which is not
> +	 * reference counted.
> +	 */
> +	tsk->set_child_tid = NULL;
> +

Can't we just move both

	p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
	/*
	 * Clear TID on mm_release()?
	 */
	p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL;

lines here?

Oleg.