Messages in this thread | | | Subject | Re: [PATCH] zram: set physical queue limits to avoid array out of bounds accesses | From | Johannes Thumshirn <> | Date | Tue, 7 Mar 2017 10:51:45 +0100 |
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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@suse.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? >>> >> 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
-- Johannes Thumshirn Storage jthumshirn@suse.de +49 911 74053 689 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: Felix Imendörffer, Jane Smithard, Graham Norton HRB 21284 (AG Nürnberg) Key fingerprint = EC38 9CAB C2C4 F25D 8600 D0D0 0393 969D 2D76 0850
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