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Subject: Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.
To:     Matthew Wilcox <willy@infradead.org>, Rich Felker <dalias@libc.org>
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 <20180323190618.GA23763@bombadil.infradead.org>
From:   Rob Landley <rob@landley.net>
Message-ID: <7e41ef7a-0bac-02fe-21fd-a1ed86c22230@landley.net>
Date:   Tue, 27 Mar 2018 23:50:02 -0500
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On 03/23/2018 02:06 PM, Matthew Wilcox wrote:
> On Fri, Mar 23, 2018 at 02:00:24PM -0400, Rich Felker wrote:
>> On Fri, Mar 23, 2018 at 05:48:06AM -0700, Matthew Wilcox 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.
>>
>> Generally libc does not have a view of the current vm maps, and thus
>> in passing "random numbers", they would have to be uniform across the
>> whole vm space and thus non-uniform once the kernel rounds up to avoid
>> existing mappings.
> 
> I'm aware that you're the musl author, but glibc somehow manages to
> provide etext, edata and end, demonstrating that it does know where at
> least some of the memory map lies.

You can parse /proc/self/maps, but it's really expensive and disgusting.

Rob