KASLR support on ARM with Kernel 4.9 and 4.14

[Ard Biesheuvel]

On Fri, 25 Sep 2020 at 22:28, Kees Cook <keescook at chromium.org> wrote:
>
> On Fri, Sep 25, 2020 at 08:33:59PM +0530, Pintu Agarwal wrote:
> > This is regarding the KASLR feature support on ARM for the kernel
> > version 4.9 and 4.14.
> >
> > Is KASLR supported on ARM-32 Linux 4.9 and above ?
>
> Sorry, this feature did not yet land in upstream:
> https://github.com/KSPP/linux/issues/3
>
> Here was the earlier effort:
> https://lore.kernel.org/kernel-hardening/20170814125411.22604-1-ard.biesheuvel@linaro.org/
>
> > Is it dependent on CONFIG_RANDOMIZE_BASE or
>
> CONFIG_RANDOMIZE_BASE is what is used on other architectures to control
> the feature.
>
> > /proc/sys/kernel/randomize_va_space ?
> > Is there any relation between these two?
>
> No, the latter is about userspace addresses.
>
> > Is the changing kernel symbols (in every boot), only possible if KASLR
> > is enabled, or there is another way it can happen?
>
> I think you meant kernel symbol addresses (not the symbols themselves).
> But yes, I wouldn't expect the addresses to move if you didn't either
> rebuild the kernel or had something else moving the kernel at boot (i.e.
> the boot loader).
>
> > I have these queries because,
> > In one of the arm-32 devices with Kernel 4.14, I observed that
> > CONFIG_RANDOMIZE_BASE is not available.
> > But /proc/sys/kernel/randomize_va_space is set to 2.
> > However, I also observed that symbol addresses are changing in every boot.
> >
> > 1st boot cycle:
> > [root ~]# cat /proc/kallsyms | grep "sys_open"
> > a5b4de92 T sys_open
> > [root at sa515m ~]#
> >
> > 2nd boot cycle:
> > [root ~]# cat /proc/kallsyms | grep "sys_open"
> > f546ed66 T sys_open
> >
> > So, I am wondering how this is possible without KASLR
> > (CONFIG_RANDOMIZE_BASE) support in Kernel ?
>

Those addresses were obfuscated by kptr_restrict