Re: [PATCH v2] x86, kaslr: propagate base load address calculation

From: Jiri Kosina
Date: Tue Feb 17 2015 - 07:21:27 EST

On Tue, 17 Feb 2015, Borislav Petkov wrote:

> > Commit e2b32e678 ("x86, kaslr: randomize module base load address") makes
> > the base address for module to be unconditionally randomized in case when
> > CONFIG_RANDOMIZE_BASE is defined and "nokaslr" option isn't present on the
> > commandline.
> >
> > This is not consistent with how choose_kernel_location() decides whether
> > it will randomize kernel load base.
> >
> > Namely, CONFIG_HIBERNATION disables kASLR (unless "kaslr" option is
> > explicitly specified on kernel commandline), which makes the state space
> > larger than what module loader is looking at. IOW CONFIG_HIBERNATION &&
> > CONFIG_RANDOMIZE_BASE is a valid config option, kASLR wouldn't be applied
> > by default in that case, but module loader is not aware of that.
> >
> > Instead of fixing the logic in module.c, this patch takes more generic
> > aproach. It introduces a new bootparam setup data_type SETUP_KASLR and
> > uses that to pass the information whether kaslr has been applied during
> > kernel decompression, and sets a global 'kaslr_enabled' variable
> > accordingly, so that any kernel code (module loading, livepatching, ...)
> > can make decisions based on its value.
> >
> > x86 module loader is converted to make use of this flag.
> >
> > Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
> > ---
> >
> > v1 -> v2:
> >
> > Originally I just calculated the fact on the fly from difference between
> > __START_KERNEL and &text, but Kees correctly pointed out that this doesn't
> > properly catch the case when the offset is randomized to zero. I don't see
> Yeah, about that. I think we want to do the thing in addition so that
> we don't have the misleading "Kernel Offset:..." line in splats in case
> kaslr is off.
> Right?

I don't have strong feelings either way. It seems slightly nicer to have a
predictable oops output format no matter the CONFIG_ options and
command-line contents, but if you feel like seeing the 'Kernel offset: 0'
in 'nokaslr' and !CONFIG_RANDOMIZE_BASE cases is unnecessary noise, feel
free to make this change to my patch.


Jiri Kosina
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