Re: [RFC PATCH] Randomization of address chosen by mmap.

[Ilya Smith <blackzert@gmail.com>]

> On 5 Mar 2018, at 22:47, Matthew Wilcox <willy@infradead.org> wrote:
>>>> - the entropy you provide is like 16 bit, that is really not so hard to brute
>>> 
>>> It's 16 bits per mapping.  I think that'll make enough attacks harder
>>> to be worthwhile.
>> 
>> Well yes, its ok, sorry. I just would like to have 32 bit entropy maximum some day :)
> 
> We could put 32 bits of padding into the prot argument on 64-bit systems
> (and obviously you need a 64-bit address space to use that many bits).  The
> thing is that you can't then put anything else into those pages (without
> using MAP_FIXED).
> 

This one sounds good to me. In my approach it is possible to map there, but ok.

>>>> - if you unmap/remap one page inside region, field vma_guard will show head 
>>>> or tail pages for vma, not both; kernel don’t know how to handle it
>>> 
>>> There are no head pages.  The guard pages are only placed after the real end.
>> 
>> Ok, we have MG where G = vm_guard, right? so when you do vm_split, 
>> you may come to situation - m1g1m2G, how to handle it? I mean when M is 
>> split with only one page inside this region. How to handle it?
> 
> I thought I covered that in my earlier email.  Using one letter per page,
> and a five-page mapping with two guard pages: MMMMMGG.  Now unmap the
> fourth page, and the VMA gets split into two.  You get: MMMGMGG.
> 
I was just interesting, it’s not the issue to me. Now its clear, thanks.

>>> I can't agree with that.  The user has plenty of opportunities to get
>>> randomness; from /dev/random is the easiest, but you could also do timing
>>> attacks on your own cachelines, for example.
>> 
>> I think the usual case to use randomization for any mmap or not use it at all 
>> for whole process. So here I think would be nice to have some variable 
>> changeable with sysctl (root only) and ioctl (for greedy processes).
> 
> I think this functionality can just as well live inside libc as in
> the kernel.
> 

Good news for them :)

>> Well, let me summary:
>> My approach chose random gap inside gap range with following strings:
>> 
>> +	addr = get_random_long() % ((high - low) >> PAGE_SHIFT);
>> +	addr = low + (addr << PAGE_SHIFT);
>> 
>> Could be improved limiting maximum possible entropy in this shift.
>> To prevent situation when attacker may massage allocations and 
>> predict chosen address, I randomly choose memory region. I’m still
>> like my idea, but not going to push it anymore, since you have yours now.
>> 
>> Your idea just provide random non-mappable and non-accessable offset
>> from best-fit region. This consumes memory (1GB gap if random value 
>> is 0xffff). But it works and should work faster and should resolve the issue.
> 
> umm ... 64k * 4k is a 256MB gap, not 1GB.  And it consumes address space,
> not memory.
> 

hmm, yes… I found 8 bits somewhere.. 256MB should be enough for everyone.

>> My point was that current implementation need to be changed and you
>> have your own approach for that. :)
>> Lets keep mine in the mind till better times (or worse?) ;)
>> Will you finish your approach and upstream it?
> 
> I'm just putting it out there for discussion.  If people think this is
> the right approach, then I'm happy to finish it off.  If the consensus
> is that we should randomly pick addresses instead, I'm happy if your
> approach gets merged.

So now, its time to call for people? Sorry, I’m new here.

Thanks,
Ilya




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