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Subject: Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.
From:   Ilya Smith <blackzert@gmail.com>
In-Reply-To: <20180323124806.GA5624@bombadil.infradead.org>
Date:   Fri, 23 Mar 2018 20:55:49 +0300
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 <20180323124806.GA5624@bombadil.infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
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> On 23 Mar 2018, at 15:48, Matthew Wilcox <willy@infradead.org> wrote:
>=20
> 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.
>=20
> 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;
2. User mode is not that layer which should be responsible for choosing
random address or handling entropy;
3. Memory fragmentation is unpredictable in this case

Off course user mode could use random =E2=80=98hint=E2=80=99 address, =
but kernel may
discard this address if it is occupied for example and allocate just =
before
closest vma. So this solution doesn=E2=80=99t give that much security =
like=20
randomization address inside kernel.=