KASLR support on ARM with Kernel 4.9 and 4.14
pintu.ping at gmail.com
Mon Sep 28 09:45:23 EDT 2020
On Sat, 26 Sep 2020 at 22:10, Kees Cook <keescook at chromium.org> wrote:
> > >> 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).
> Okay, so yes. And this appears to have the hashing of %p backported. I
> cannot, however, explain why it's showing hashed pointers instead of
> just NULL, though.
> It might be related to these commits but they're not in that kernel:
> 3e5903eb9cff ("vsprintf: Prevent crash when dereferencing invalid pointers")
> 7bd57fbc4a4d ("vsprintf: don't obfuscate NULL and error pointers")
> > ==> 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 ?
> Try setting kptr_restrict to 0 and see if the symbol addresses change? I
> suspect Ard is correct: there's no KASLR here, just hashed pointers
> behaving weird on an old non-stock kernel. :)
Okay. Thank you so much for your comments and suggestions.
You mean to say, setting kptr_restrict to 0 may avoid changing symbol
addresses in 4.14 ?
And, sorry, I could not understand the thing about this "hashed pointers".
How can I check this behavior in source code to understand better?
Is it possible to give some reference ?
I wanted to disable this hash pointer on 4.14 kernel and check the behavior.
Also if possible, we would like to make this similar change on 4.9
kernel as well.
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