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

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Message-ID: <20180305162343.GA8230@bombadil.infradead.org>
Date:   Mon, 5 Mar 2018 08:23:43 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Ilya Smith <blackzert@...il.com>
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 Mon, Mar 05, 2018 at 04:09:31PM +0300, Ilya Smith wrote:
> > 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

Umm ... yes, each time you call mmap, you get a VMA.  I'm not sure why
that's a problem with my patch.  I was trying to solve the problem Daniel
pointed out, that mapping a guard region after each mmap cost twice as
many VMAs, and it solves that problem.

> - now feature vma_merge shouldn’t work at all, until MAP_FIXED is set or
> PROT_GUARD(0)

That's true.

> - 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.

> - in your patch you don’t use vm_guard at address searching, I see many roots 
> of bugs here

Don't need to.  vm_end includes the guard pages.

> - 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.

> - 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 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.