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

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Message-ID: <alpine.LNX.2.00.1502171319040.2279@pobox.suse.cz>
Date:	Tue, 17 Feb 2015 13:21:20 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Borislav Petkov <bp@...en8.de>
cc:	Kees Cook <keescook@...omium.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	live-patching@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v2] x86, kaslr: propagate base load address calculation

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@...e.cz>
> > ---
> > 
> > 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.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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