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Date: Mon, 5 Mar 2018 16:09:31 +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 4 Mar 2018, at 23:56, Matthew Wilcox <willy@...radead.org> wrote:
> Thinking about this more ...
>
> - When you call munmap, if you pass in the same (addr, length) that were
> used for mmap, then it should unmap the guard pages as well (that
> wasn't part of the patch, so it would have to be added)
> - If 'addr' is higher than the mapped address, and length at least
> reaches the end of the mapping, then I would expect the guard pages to
> "move down" and be after the end of the newly-shortened mapping.
> - If 'addr' is higher than the mapped address, and the length doesn't
> reach the end of the old mapping, we split the old mapping into two.
> I would expect the guard pages to apply to both mappings, insofar as
> they'll fit. For an example, suppose we have a five-page mapping with
> two guard pages (MMMMMGG), and then we unmap the fourth page. Now we
> have a three-page mapping with one guard page followed immediately
> by a one-page mapping with two guard pages (MMMGMGG).
I’m analysing that approach and see much more problems:
- each time you call mmap like this, you still increase count of vmas as my
patch did
- now feature vma_merge shouldn’t work at all, until MAP_FIXED is set or
PROT_GUARD(0)
- the entropy you provide is like 16 bit, that is really not so hard to brute
- in your patch you don’t use vm_guard at address searching, I see many roots
of bugs here
- 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
- user mode now choose entropy with PROT_GUARD macro, where did he gets it?
User mode shouldn’t be responsible for entropy at all
I can’t understand what direction this conversation is going to. I was talking
about weak implementation in Linux kernel but got many comments about ASLR
should be implemented in user mode what is really weird to me.
I think it is possible to add GUARD pages into my implementations, but initially
problem was about entropy of address choosing. I would like to resolve it step by
step.
Thanks,
Ilya
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