linux-kernel - Re: [PATCH] mm: optionally disable brk()

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Date:   Mon, 5 Oct 2020 11:55:21 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Topi Miettinen <toiwoton@...il.com>, Michal Hocko <mhocko@...e.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: optionally disable brk()

On 05.10.20 11:47, Topi Miettinen wrote:
> On 5.10.2020 12.13, David Hildenbrand wrote:
>> On 05.10.20 08:12, Michal Hocko wrote:
>>> On Sat 03-10-20 00:44:09, Topi Miettinen wrote:
>>>> On 2.10.2020 20.52, David Hildenbrand wrote:
>>>>> On 02.10.20 19:19, Topi Miettinen wrote:
>>>>>> The brk() system call allows to change data segment size (heap). This
>>>>>> is mainly used by glibc for memory allocation, but it can use mmap()
>>>>>> and that results in more randomized memory mappings since the heap is
>>>>>> always located at fixed offset to program while mmap()ed memory is
>>>>>> randomized.
>>>>>
>>>>> Want to take more Unix out of Linux?
>>>>>
>>>>> Honestly, why care about disabling? User space can happily use mmap() if
>>>>> it prefers.
>>>>
>>>> brk() interface doesn't seem to be used much and glibc is happy to switch to
>>>> mmap() if brk() fails, so why not allow disabling it optionally? If you
>>>> don't care to disable, don't do it and this is even the default.
>>>
>>> I do not think we want to have config per syscall, do we?
>>
>> I do wonder if grouping would be a better option then (finding a proper
>> level of abstraction ...).
> 
> If hardening and compatibility are seen as tradeoffs, perhaps there 
> could be a top level config choice (CONFIG_HARDENING_TRADEOFF) for this. 
> It would have options
> - "compatibility" (default) to gear questions for maximum compatibility, 
> deselecting any hardening options which reduce compatibility
> - "hardening" to gear questions for maximum hardening, deselecting any 
> compatibility options which reduce hardening
> - "none/manual": ask all questions like before

I think the general direction is to avoid an exploding set of config
options. So if there isn't a *real* demand, I guess gluing this to a
single option ("CONFIG_SECURITY_HARDENING") might be good enough.

-- 
Thanks,

David / dhildenb