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

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Message-Id: <7FA6631B-951F-42F4-A7BF-8E5BB734D709@gmail.com>
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