linux-kernel - Re: [PATCH] avoid return NULL on root rb_node in rb_next/rb

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Date:	Mon, 28 Jun 2010 15:55:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	shenghui <crosslonelyover@...il.com>
Cc:	kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
	Greg KH <greg@...ah.com>, linux-mm@...ck.org, mingo@...e.hu
Subject: Re: [PATCH] avoid return NULL on root rb_node in rb_next/rb_prev
 in lib/rbtree.c

On Mon, 2010-06-28 at 21:17 +0800, shenghui wrote:
> Hi,
> 
>        I'm reading cfs code, and get the following potential bug.
> 
> In kernel/sched_fair.c, we can get the following call thread:
> 
> 1778static struct task_struct *pick_next_task_fair(struct rq *rq)
> 1779{
> ...
> 1787        do {
> 1788                se = pick_next_entity(cfs_rq);
> 1789                set_next_entity(cfs_rq, se);
> 1790                cfs_rq = group_cfs_rq(se);
> 1791        } while (cfs_rq);
> ...
> 1797}
> 
>  925static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
>  926{
>  927        struct sched_entity *se = __pick_next_entity(cfs_rq);
> ...
>  941        return se;
>  942}
> 
>  377static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq)
>  378{
>  379        struct rb_node *left = cfs_rq->rb_leftmost;
>  380
>  381        if (!left)
>  382                return NULL;
>  ...
>  385}
> 
> To manipulate cfs_rq->rb_leftmost, __dequeue_entity does the following:
> 
>  365static void __dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  366{
>  367        if (cfs_rq->rb_leftmost == &se->run_node) {
>  368                struct rb_node *next_node;
>  369
>  370                next_node = rb_next(&se->run_node);
>  371                cfs_rq->rb_leftmost = next_node;
>  372        }
>  373
>  374        rb_erase(&se->run_node, &cfs_rq->tasks_timeline);
>  375}
> 
> Here, if se->run_node is the root rb_node, next_node will be set NULL
> by rb_next.
> Then __pick_next_entity may get NULL on some call, and set_next_entity
> may deference
> NULL value.

So if ->rb_leftmost is NULL, then the if (!left) check in
__pick_next_entity() would return null.

As to the NULL deref in in pick_next_task_fair()->set_next_entity() that
should never happen because pick_next_task_fair() will bail
on !->nr_running.

Furthermore, you've failed to mention what kernel version you're looking
at.
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