linux-kernel - Re: [PATCH] zram: set physical queue limits to avoid array out of bounds accesses

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Date:   Tue, 7 Mar 2017 16:23:43 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Hannes Reinecke <hare@...e.de>
Cc:     Johannes Thumshirn <jthumshirn@...e.de>, Jens Axboe <axboe@...com>,
        Nitin Gupta <ngupta@...are.org>,
        Christoph Hellwig <hch@....de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        yizhan@...hat.com,
        Linux Block Layer Mailinglist <linux-block@...r.kernel.org>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] zram: set physical queue limits to avoid array out of
 bounds accesses

Hi Hannes,

On Tue, Mar 7, 2017 at 4:00 PM, Hannes Reinecke <hare@...e.de> wrote:
> On 03/07/2017 06:22 AM, Minchan Kim wrote:
>> Hello Johannes,
>>
>> On Mon, Mar 06, 2017 at 11:23:35AM +0100, Johannes Thumshirn wrote:
>>> zram can handle at most SECTORS_PER_PAGE sectors in a bio's bvec. When using
>>> the NVMe over Fabrics loopback target which potentially sends a huge bulk of
>>> pages attached to the bio's bvec this results in a kernel panic because of
>>> array out of bounds accesses in zram_decompress_page().
>>
>> First of all, thanks for the report and fix up!
>> Unfortunately, I'm not familiar with that interface of block layer.
>>
>> It seems this is a material for stable so I want to understand it clear.
>> Could you say more specific things to educate me?
>>
>> What scenario/When/How it is problem?  It will help for me to understand!
>>

Thanks for the quick response!

> The problem is that zram as it currently stands can only handle bios
> where each bvec contains a single page (or, to be precise, a chunk of
> data with a length of a page).

Right.

>
> This is not an automatic guarantee from the block layer (who is free to
> send us bios with arbitrary-sized bvecs), so we need to set the queue
> limits to ensure that.

What does it mean "bios with arbitrary-sized bvecs"?
What kinds of scenario is it used/useful?

And how can we solve it by setting queue limit?

Sorry for the many questions due to limited knowledge.

Thanks.