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

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Date:	Wed, 17 Oct 2012 09:47:04 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>
Cc:	Tejun Heo <tj@...nel.org>,
	"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 Wed, Oct 17, 2012 at 09:02:22AM +0900, Jun'ichi Nomura wrote:
> 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.

If q->root_blkg is NULL, will it not lead to NULL pointer dereference.
(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.

I would rather prefer to check for this boundary condition early and
return instead of letting it fall through all these conditions and
then figure out yes we have no next rl. IMO, code becomes easier to
understand if nothing else. Otherwise one needs a step by step 
explanation as above to show that case of q->root_blkg is covered.

Thanks
Vivek
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