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

[Johannes Thumshirn]

On 03/07/2017 09:55 AM, Minchan Kim wrote:
> On Tue, Mar 07, 2017 at 08:48:06AM +0100, Hannes Reinecke wrote:
>> On 03/07/2017 08:23 AM, Minchan Kim wrote:
>>> Hi Hannes,
>>>
>>> On Tue, Mar 7, 2017 at 4:00 PM, Hannes Reinecke <hare@xxxxxxx> 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?
>>>
>> Each bio contains a list of bvecs, each of which points to a specific
>> memory area:
>>
>> struct bio_vec {
>> 	struct page	*bv_page;
>> 	unsigned int	bv_len;
>> 	unsigned int	bv_offset;
>> };
>>
>> The trick now is that while 'bv_page' does point to a page, the memory
>> area pointed to might in fact be contiguous (if several pages are
>> adjacent). Hence we might be getting a bio_vec where bv_len is _larger_
>> than a page.
> 
> Thanks for detail, Hannes!
> 
> If I understand it correctly, it seems to be related to bid_add_page
> with high-order page. Right?
> 
> If so, I really wonder why I don't see such problem because several
> places have used it and I expected some of them might do IO with
> contiguous pages intentionally or by chance. Hmm,
> 
> IIUC, it's not a nvme specific problme but general problem which
> can trigger normal FSes if they uses contiguos pages?
> 

I'm not a FS expert, but a quick grep shows that non of the file-systems
does the

for_each_sg()
	while(bio_add_page()))

trick NVMe does.

>>
>> Hence the check for 'is_partial_io' in zram_drv.c (which just does a
>> test 'if bv_len != PAGE_SIZE) is in fact wrong, as it would trigger for
>> partial I/O (ie if the overall length of the bio_vec is _smaller_ than a
>> page), but also for multipage bvecs (where the length of the bio_vec is
>> _larger_ than a page).
> 
> Right. I need to look into that. Thanks for the pointing out!
> 
>>
>> So rather than fixing the bio scanning loop in zram it's easier to set
>> the queue limits correctly so that 'is_partial_io' does the correct
>> thing and the overall logic in zram doesn't need to be altered.
> 
> 
> Isn't that approach require new bio allocation through blk_queue_split?
> Maybe, it wouldn't make severe regression in zram-FS workload but need
> to test.

Yes, but blk_queue_split() needs information how big the bvecs can be,
hence the patch.

For details have a look into blk_bio_segment_split() in block/blk-merge.c

It get's the max_sectors from blk_max_size_offset() which is
q->limits.max_sectors when q->limits.chunk_sectors isn't set and
then loops over the bio's bvecs to check when to split the bio and then
calls bio_split() when appropriate.

> 
> Is there any ways to trigger the problem without real nvme device?
> It would really help to test/measure zram.

It isn't a /real/ device but the fabrics loopback target. If you want a
fast reproducible test-case, have a look at:

https://github.com/ddiss/rapido/
the cut_nvme_local.sh script set's up the correct VM for this test. Then
a simple mkfs.xfs /dev/nvme0n1 will oops.

> 
> Anyway, to me, it's really subtle at this moment so I doubt it should
> be stable material. :(

I'm not quite sure, it's at least 4.11 material. See above.

Thanks,
	Johannes


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Johannes Thumshirn                                          Storage
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