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Message-ID: <CAJcbSZFTX3uiS2g8JriS6+z_+WrG8z3hrQo4OSuyHpiyUDJWYA@mail.gmail.com>
Date: Fri, 11 Aug 2017 08:09:04 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Herbert Xu <herbert@...dor.apana.org.au>, "David S . Miller" <davem@...emloft.net>,
Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
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Subject: Re: x86: PIE support and option to extend KASLR randomization
On Fri, Aug 11, 2017 at 5:41 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Thomas Garnier <thgarnie@...gle.com> wrote:
>
>> Changes:
>> - v2:
>> - Add support for global stack cookie while compiler default to fs without
>> mcmodel=kernel
>> - Change patch 7 to correctly jump out of the identity mapping on kexec load
>> preserve.
>>
>> These patches make the changes necessary to build the kernel as Position
>> Independent Executable (PIE) on x86_64. A PIE kernel can be relocated below
>> the top 2G of the virtual address space. It allows to optionally extend the
>> KASLR randomization range from 1G to 3G.
>
> So this:
>
> 61 files changed, 923 insertions(+), 299 deletions(-)
>
> ... is IMHO an _awful_ lot of churn and extra complexity in pretty fragile pieces
> of code, to gain what appears to be only ~1.5 more bits of randomization!
The range increase is a way to use PIE right away.
>
> Do these changes get us closer to being able to build the kernel as truly position
> independent, i.e. to place it anywhere in the valid x86-64 address space? Or any
> other advantages?
Yes, PIE allows us to put the kernel anywhere in memory. It will allow
us to have a full randomized address space where position and order of
sections are completely random. There is still some work to get there
but being able to build a PIE kernel is a significant step.
>
> Thanks,
>
> Ingo
--
Thomas
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