linux-kernel - Re: [PATCH] mm: fix invalid node in alloc_migrate

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Date:	Thu, 31 Mar 2016 15:13:41 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Xishi Qiu <qiuxishi@...wei.com>
Cc:	Joonsoo Kim <js1304@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Laura Abbott <lauraa@...eaurora.org>, zhuhui@...omi.com,
	wangxq10@....edu.cn, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: fix invalid node in alloc_migrate_target()

On 03/29/2016 03:06 PM, Vlastimil Babka wrote:
> On 03/25/2016 08:22 PM, Andrew Morton wrote:
>> Also, mm/mempolicy.c:offset_il_node() worries me:
>>
>> 	do {
>> 		nid = next_node(nid, pol->v.nodes);
>> 		c++;
>> 	} while (c <= target);
>>
>> Can't `nid' hit MAX_NUMNODES?
>
> AFAICS it can. interleave_nid() uses this and the nid is then used e.g.
> in node_zonelist() where it's used for NODE_DATA(nid). That's quite
> scary. It also predates git. Why don't we see crashes or KASAN finding this?

Ah, I see. In offset_il_node(), nid is initialized to -1, and the number 
of do-while iterations calling next_node() is up to the number of bits 
set in the pol->v.nodes bitmap, so it can't reach past the last set bit 
and return MAX_NUMNODES.