linux-kernel - Re: [PATCH] Fix use-after-free of q->root_blkg and q->root

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Date:	Wed, 17 Oct 2012 09:02:22 +0900
From:	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Vivek Goyal <vgoyal@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Fix use-after-free of q->root_blkg and q->root_rl.blkg

On 10/17/12 08:20, Tejun Heo wrote:
>>>> -	if (ent == &q->root_blkg->q_node)
>>>> +	if (q->root_blkg && ent == &q->root_blkg->q_node)
>>>
>>> Can we fix it little differently. Little earlier in the code, we check for
>>> if q->blkg_list is empty, then all the groups are gone, and there are
>>> no more request lists hence and return NULL.
>>>
>>> Current code:
>>>         if (rl == &q->root_rl) {
>>>                 ent = &q->blkg_list;
>>>
>>> Modified code:
>>>         if (rl == &q->root_rl) {
>>>                 ent = &q->blkg_list;
>>> 		/* There are no more block groups, hence no request lists */
>>> 		if (list_empty(ent))
>>> 			return NULL;
>>> 	}
> 
> Do we need this at all?  q->root_blkg being NULL is completely fine
> there and the comparison would work as expected, no?

Hmm?

If list_empty(ent) and q->root_blkg == NULL,

>         /* walk to the next list_head, skip root blkcg */
>         ent = ent->next;

ent is &q->blkg_list again.

>         if (ent == &q->root_blkg->q_node)

So ent is not &q->root_blkg->q_node.

>                 ent = ent->next;
>         if (ent == &q->blkg_list)
>                 return NULL;

And we return NULL here.

Ah, yes. You are correct.
We can do without the above hunk.

-- 
Jun'ichi Nomura, NEC Corporation

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