KASLR support on ARM with Kernel 4.9 and 4.14

[Kees Cook]

On Fri, Sep 25, 2020 at 11:09:10PM +0200, Ard Biesheuvel wrote:
> On Fri, 25 Sep 2020 at 22:47, Kees Cook <keescook at chromium.org> wrote:
> >
> > 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).
> >
> 
> Ah yes, good point. But it does look suspiciously like they are being
> mangled in a similar way.
> 
> 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...

-- 
Kees Cook