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Date: Mon, 5 Mar 2018 23:20:31 +0300
From: Ilya Smith <blackzert@...il.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Daniel Micay <danielmicay@...il.com>,
Kees Cook <keescook@...omium.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Dan Williams <dan.j.williams@...el.com>,
Michal Hocko <mhocko@...e.com>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Jan Kara <jack@...e.cz>,
Jerome Glisse <jglisse@...hat.com>,
Hugh Dickins <hughd@...gle.com>,
Helge Deller <deller@....de>,
Andrea Arcangeli <aarcange@...hat.com>,
Oleg Nesterov <oleg@...hat.com>,
Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC PATCH] Randomization of address chosen by mmap.
> On 5 Mar 2018, at 22:47, Matthew Wilcox <willy@...radead.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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