Re: [00/41] Large Blocksize Support V7 (adds memmap support)

[Mel Gorman <mel@csn.ul.ie>]

On Fri, 2007-09-14 at 18:10 +0200, Goswin von Brederlow wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> writes:
> 
> > In my attack, I cause the kernel to allocate lots of unmovable allocations
> > and deplete movable groups. I theoretically then only need to keep a
> > small number (1/2^N) of these allocations around in order to DoS a
> > page allocation of order N.
> 
> I'm assuming that when an unmovable allocation hijacks a movable group
> any further unmovable alloc will evict movable objects out of that
> group before hijacking another one. right?
> 

No eviction takes place. If an unmovable allocation gets placed in a
movable group, then steps are taken to ensure that future unmovable
allocations will take place in the same range (these decisions take
place in __rmqueue_fallback()). When choosing a movable block to
pollute, it will also choose the lowest possible block in PFN terms to
steal so that fragmentation pollution will be as confined as possible.
Evicting the unmovable pages would be one of those expensive steps that
have been avoided to date.

> > And it doesn't even have to be a DoS. The natural fragmentation
> > that occurs today in a kernel today has the possibility to slowly push out
> > the movable groups and give you the same situation.
> 
> How would you cause that? Say you do want to purposefully place one
> unmovable 4k page into every 64k compund page. So you allocate
> 4K. First 64k page locked. But now, to get 4K into the second 64K page
> you have to first use up all the rest of the first 64k page. Meaning
> one 4k chunk, one 8k chunk, one 16k cunk, one 32k chunk. Only then
> will a new 64k chunk be broken and become locked.

It would be easier early in the boot to mmap a large area and fault it
in in virtual address order then mlock every a page every 64K. Early in
the systems lifetime, there will be a rough correlation between physical
and virtual memory.

Without mlock(), the most successful attack will like mmap() a 60K
region and fault it in as an attempt to get pagetable pages placed in
every 64K region. This strategy would not work with grouping pages by
mobility though as it would group the pagetable pages together.

Targetted attacks on grouping pages by mobility are not very easy and
not that interesting either. As Nick suggests, the natural fragmentation
over long periods of time is what is interesting.

> So to get the last 64k chunk used all previous 32k chunks need to be
> blocked and you need to allocate 32k (or less if more is blocked). For
> all previous 32k chunks to be blocked every second 16k needs to be
> blocked. To block the last of those 16k chunks all previous 8k chunks
> need to be blocked and you need to allocate 8k. For all previous 8k
> chunks to be blocked every second 4k page needs to be used. To alloc
> the last of those 4k pages all previous 4k pages need to be used.
> 
> So to construct a situation where no continious 64k chunk is free you
> have to allocate <total mem> - 64k - 32k - 16k - 8k - 4k (or there
> about) of memory first. Only then could you free memory again while
> still keeping every 64k page blocked. Does that occur naturally given
> enough ram to start with?
> 

I believe it's very difficult to craft an attack that will work in a
short period of time. An attack that worked on 2.6.22 as well may have
no success on 2.6.23-rc4-mm1 for example as grouping pages by mobility
does it make it exceedingly hard to craft an attack unless the attacker
can mlock large amounts of memory.

> 
> Too see how bad fragmentation could be I wrote a little progamm to
> simulate allocations with the following simplified alogrithm:
> 
> Memory management:
> - Free pages are kept in buckets, one per order, and sorted by address.
> - alloc() the front page (smallest address) out of the bucket of the
>   right order or recursively splits the next higher bucket.
> - free() recursively tries to merge a page with its neighbour and puts
>   the result back into the proper bucket (sorted by address).
> 
> Allocation and lifetime:
> - Every tick a new page is allocated with random order.

This step in itself is not representative of what happens in the kernel.
The vast vast majority of allocations are order-0. It's a fun analysis
but I'm not sure can we draw any conclusions from it.

Statistical analysis of the buddy algorithm have implied that it doesn't
suffer that badly from external fragmentation but we know in practice
that things are different. A model is hard because minimally the
lifetime of pages varies widely.

> - The order is a triangle distribution with max at 0 (throw 2 dice,
>   add the eyes, subtract 7, abs() the number).
> - The page is scheduled to be freed after X ticks. Where X is nearly
>   a gaus curve centered at 0 and maximum at <total num pages> * 1.5.
>   (What I actualy do is throw 8 dice and sum them up and shift the
>    result.)
> 

I doubt this is how the kernel behaves either.

> Display:
> I start with a white window. Every page allocation draws a black box
> from the address of the page and as wide as the page is big (-1 pixel to
> give a seperation to the next page). Every page free draws a yellow
> box in place of the black one. Yellow to show where a page was in use
> at one point while white means the page was never used.
> 
> As the time ticks the memory fills up. Quickly at first and then comes
> to a stop around 80% filled. And then something interesting
> happens. The yellow regions (previously used but now free) start
> drifting up. Small pages tend to end up in the lower addresses and big
> pages at the higher addresses. The memory defragments itself to some
> degree.
> 
> http://mrvn.homeip.net/fragment/
> 
> Simulating 256MB ram and after 1472943 ticks and 530095 4k, 411841 8k,
> 295296 16k, 176647 32k and 59064 64k allocations you get this:
> http://mrvn.homeip.net/fragment/256mb.png
> 
> Simulating 1GB ram and after 5881185 ticks  and 2116671 4k, 1645957
> 8k, 1176994 16k, 705873 32k and 235690 64k allocations you get this:
> http://mrvn.homeip.net/fragment/1gb.png
> 

These type of pictures feel somewhat familiar
(http://www.skynet.ie/~mel/anti-frag/2007-02-28/page_type_distribution.jpg).

-- 
Mel Gorman