linux-kernel - Re: KASLR support on ARM with Kernel 4.9 and 4.14

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Date:   Sat, 26 Sep 2020 13:28:02 +0530
From:   Pintu Agarwal <pintu.ping@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Dave Martin <dave.martin@....com>,
        Kernelnewbies <kernelnewbies@...nelnewbies.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        open list <linux-kernel@...r.kernel.org>,
        Tony Lindgren <tony@...mide.com>, matt@...eblueprint.co.uk,
        nico@...aro.org, Thomas Garnier <thgarnie@...gle.com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: KASLR support on ARM with Kernel 4.9 and 4.14

On Sat, 26 Sep 2020 at 05:17, Kees Cook <keescook@...omium.org> wrote:
> >
> > For a 3/1 split ARM kernel of the typical size, all kernel virtual
> > addresses start with 0xc0, and given that the kernel is located at the
> > start of the linear map, those addresses cannot change even if you
> > move the kernel around in physical memory.
>
> I wonder if this is an Android Common kernel? I think there was %p
> hashing in there before v4.15, but with a different implementation...
>

Hi,
Thank you all for all your reply and comments so far!
Here are some follow-up replies.

>> What device is this? Is it a stock kernel?
This is a Qualcomm Snapdragon Automotive board one with Linux Kernel
4.9 and one with 4.14.

>> Is the boot loader changing the base address? (What boot loader are you
>> using?)
Ohh I did not knew that the bootloader can also change the base address.
I think it uses UEFI.
How to check if bootloader is doing this ?
BTW, both 4.9 board and 4.14 board, uses same bootloader.

>> I wonder if this is an Android Common kernel?
It uses the below kernel for 4.14:
https://gitlab.com/quicla/kernel/msm-4.14/-/tree/LE.UM.3.4.2.r1.5  (or
similar branch).
==> The case where symbol addresses are changing.

kptr_restrict is set to 2 by default:
/ # cat /proc/sys/kernel/kptr_restrict
2

Basically, the goal is:
* To understand how addresses are changing in 4.14 Kernel (without
KASLR support)?
* Is it possible to support the same in 4.9 Kernel ?

--
Thanks,
Pintu