Re: [PATCH] Fix use-after-free of q->root_blkg and q->root_rl.blkg

From: Jun'ichi Nomura
Date: Tue Oct 16 2012 - 20:02:48 EST


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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