Subject: Re: [PATCH] mm: fix invalid node in alloc_migrate_target()
To: Andrew Morton <akpm@linux-foundation.org>, Xishi Qiu <qiuxishi@huawei.com>
References: <56F4E104.9090505@huawei.com>
 <20160325122237.4ca4e0dbca215ccbf4f49922@linux-foundation.org>
 <56FA7DC8.4000902@suse.cz>
Cc: Joonsoo Kim <js1304@gmail.com>, David Rientjes <rientjes@google.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Laura Abbott <lauraa@codeaurora.org>, zhuhui@xiaomi.com,
        wangxq10@lzu.edu.cn, Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <56FD2285.4080600@suse.cz>
Date: Thu, 31 Mar 2016 15:13:41 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.7.0
MIME-Version: 1.0
In-Reply-To: <56FA7DC8.4000902@suse.cz>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 743
Lines: 19

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.