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

[Ilya Smith <blackzert@gmail.com>]

> On 4 Mar 2018, at 23:56, Matthew Wilcox <willy@infradead.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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