KASLR support on ARM with Kernel 4.9 and 4.14

[Kees Cook]

On Fri, Sep 25, 2020 at 10:37:01PM +0200, Ard Biesheuvel wrote:
> 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

Is that true? kptr_restrict zeros (rather than hashing) the kallsyms
view. And besides, the %p hashing was added in v4.15 (but also doesn't
touch kallsyms, which does all-or-nothing to avoid breaking stuff
like perf).

-- 
Kees Cook