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

From: Borislav Petkov
Date: Tue Feb 17 2015 - 05:45:39 EST

On Fri, Feb 13, 2015 at 04:04:55PM +0100, Jiri Kosina 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.


diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index ab4734e5411d..a203da9cc445 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1275,6 +1275,9 @@ static struct notifier_block kernel_offset_notifier = {

static int __init register_kernel_offset_dumper(void)
+ if (!kaslr_enabled)
+ return 0;
return 0;


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