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

[Matthew Wilcox]

On Mon, Mar 05, 2018 at 04:09:31PM +0300, Ilya Smith wrote:
> > On 4 Mar 2018, at 23:56, Matthew Wilcox <willy@xxxxxxxxxxxxx> 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).
> 
> Iâm 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 shouldnât 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 donât 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 donât 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 shouldnât 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.