linux-kernel - Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

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Date:   Mon, 26 Mar 2018 10:46:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Ilya Smith <blackzert@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>, rth@...ddle.net,
        ink@...assic.park.msu.ru, mattst88@...il.com, vgupta@...opsys.com,
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        Helge Deller <deller@....de>, benh@...nel.crashing.org,
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Subject: Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

On Fri 23-03-18 20:55:49, Ilya Smith wrote:
> 
> > On 23 Mar 2018, at 15:48, Matthew Wilcox <willy@...radead.org> wrote:
> > 
> > On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote:
> >> Current implementation doesn't randomize address returned by mmap.
> >> All the entropy ends with choosing mmap_base_addr at the process
> >> creation. After that mmap build very predictable layout of address
> >> space. It allows to bypass ASLR in many cases. This patch make
> >> randomization of address on any mmap call.
> > 
> > Why should this be done in the kernel rather than libc?  libc is perfectly
> > capable of specifying random numbers in the first argument of mmap.
> Well, there is following reasons:
> 1. It should be done in any libc implementation, what is not possible IMO;

Is this really so helpful?

> 2. User mode is not that layer which should be responsible for choosing
> random address or handling entropy;

Why?

> 3. Memory fragmentation is unpredictable in this case
> 
> Off course user mode could use random ‘hint’ address, but kernel may
> discard this address if it is occupied for example and allocate just before
> closest vma. So this solution doesn’t give that much security like 
> randomization address inside kernel.

The userspace can use the new MAP_FIXED_NOREPLACE to probe for the
address range atomically and chose a different range on failure.

-- 
Michal Hocko
SUSE Labs