linux-kernel - Re: [PATCH v3] ipc/mqueue: Avoid relying on a stack reference past its expiry

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Date:   Sat, 8 May 2021 21:23:45 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Varad Gautam <varad.gautam@...e.com>, linux-kernel@...r.kernel.org
Cc:     Matthias von Faber <matthias.vonfaber@...-tech.de>,
        Davidlohr Bueso <dbueso@...e.de>, stable@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        James Morris <jamorris@...ux.microsoft.com>,
        Serge Hallyn <serge@...lyn.com>
Subject: Re: [PATCH v3] ipc/mqueue: Avoid relying on a stack reference past
 its expiry

Hi Varad,

On 5/7/21 3:38 PM, Varad Gautam wrote:
> @@ -1005,11 +1022,9 @@ static inline void __pipelined_op(struct wake_q_head *wake_q,
>   				  struct ext_wait_queue *this)
>   {
>   	list_del(&this->list);
> -	get_task_struct(this->task);
> -
> +	wake_q_add(wake_q, this->task);
>   	/* see MQ_BARRIER for purpose/pairing */
>   	smp_store_release(&this->state, STATE_READY);
> -	wake_q_add_safe(wake_q, this->task);
>   }
>   
>   /* pipelined_send() - send a message directly to the task waiting in

First, I was too fast: I had assumed that wake_q_add() before 
smp_store_release() would be a potential lost wakeup.

As __pipelined_op() is called within spin_lock(&info->lock), and as 
wq_sleep() will reread this->state after acquiring 
spin_lock(&info->lock), I do not see a bug anymore.

But I don't like the change: Why should ipc/*.c differ from kernel/futex.c?

--

     Manfred