[PATCH] avoid return NULL on root rb_node in rb_next/rb_prev in lib/rbtree.c

[PATCH] avoid return NULL on root rb_node in rb_next/rb_prev in lib/rbtree.c

[shenghui]

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.

 892static void
 893set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 894{
 895        /* 'current' is not kept within the tree. */
 896        if (se->on_rq) {
...
 919        se->prev_sum_exec_runtime = se->sum_exec_runtime;
 920}

Following is my patch. Please check it.

Re: [PATCH] avoid return NULL on root rb_node in rb_next/rb_prev in lib/rbtree.c

[Peter Zijlstra]

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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Re: [PATCH] avoid return NULL on root rb_node in rb_next/rb_prev in lib/rbtree.c

[shenghui]

2010/6/28 Peter Zijlstra <peterz@infradead.org>:
> 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.
>

The kernel version is 2.6.35-rc3, and 2.6.34 has the same code.

For nr->running, if current is the only process in the run queue, then
nr->running would not be zero.
1784        if (!cfs_rq->nr_running)
1785                return NULL;
pick_next_task_fair() could pass above check and run to following:
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);

Then pick_next_entity will get NULL for current is the root rb_node.
Then set_next_entity would fail on NULL deference.



-- 


Thanks and Best Regards,
shenghui

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Re: [PATCH] avoid return NULL on root rb_node in rb_next/rb_prev in lib/rbtree.c

[shenghui]

2010/6/29 shenghui <crosslonelyover@gmail.com>:
> 2010/6/28 Peter Zijlstra <peterz@infradead.org>:
>> 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.
>>
>
> The kernel version is 2.6.35-rc3, and 2.6.34 has the same code.
>
> For nr->running, if current is the only process in the run queue, then
> nr->running would not be zero.
> 1784        if (!cfs_rq->nr_running)
> 1785                return NULL;
> pick_next_task_fair() could pass above check and run to following:
> 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);
>
> Then pick_next_entity will get NULL for current is the root rb_node.
> Then set_next_entity would fail on NULL deference.
>

Sorry, I misunderstood the code. I'll put forward one new patch to
avoid the NULL condition


-- 


Thanks and Best Regards,
shenghui

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