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

[Minchan Kim]

Hi Johannes,

On Tue, Mar 07, 2017 at 10:51:45AM +0100, Johannes Thumshirn wrote:
> 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.

Aah, I see.

> 
> >>
> >> 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.

Yeb so it causes split bio which means new bio allocations which was
not needed before.

> 
> > 
> > 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.

Thanks! I will look into that.

And could you test this patch? It avoids split bio so no need new bio
allocations and makes zram code simple.