Re: [PATCH 1/4] kaslr: check user's config too when handle relocations

Baoquan He
2014-09-10T06:10:53Z

On 09/09/14 at 08:53am, Kees Cook wrote:
>
> > Hi Kees,
> >
> > Checking handle_relocations() again, I just didn't notice it's mandatory
> > to do the relocations handling in i386. So in this function delta is
> > checked to see if it's a kaslr relocation handling. This might be a
> > little confusing. But I am fine with it.
> >
> > Per your comment, you prefer to compare the output before and after
> > choose_kernel_location(). That's also good, Lu Yinghai posted a draft
> > patch in this way before, however the checking and the delta calculation
> > are not correct. I changed that and test all cases, it works well. So
> > do you like this it? If yes I will repost it.
> >
> > From 13471bd838c52a0e143c2aee81e3863cfff585bd Mon Sep 17 00:00:00 2001
> > From: Baoquan He <bhe@redhat.com>
> > Date: Mon, 25 Aug 2014 14:57:43 +0800
> > Subject: [PATCH] kaslr: check if kernel location is changed
> >
> > Signed-off-by: Baoquan He <bhe@redhat.com>
> > ---
> > arch/x86/boot/compressed/misc.c | 15 +++++++++++----
> > 1 file changed, 11 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> > index 57ab74d..887f404 100644
> > --- a/arch/x86/boot/compressed/misc.c
> > +++ b/arch/x86/boot/compressed/misc.c
> > @@ -230,8 +230,9 @@ static void error(char *x)
> > asm("hlt");
> > }
> >
> > -#if CONFIG_X86_NEED_RELOCS
> > -static void handle_relocations(void *output, unsigned long output_len)
> > +#ifdef CONFIG_X86_NEED_RELOCS
> > +static void handle_relocations(void *output_orig, void *output,
> > + unsigned long output_len)
> > {
> > int *reloc;
> > unsigned long delta, map, ptr;
> > @@ -242,6 +243,9 @@ static void handle_relocations(void *output, unsigned long output_len)
> > * Calculate the delta between where vmlinux was linked to load
> > * and where it was actually loaded.
> > */
> > + if (output_orig == output)
> > + return;
> > +
>
> I still think this needs a test for the 32-bit case, since IUIC, it
> requires relocations unconditionally.

Oops, just understood that 32 bit kernel alwasy need relocations, but
only focus on x86_64 and kaslr again when I was doing it. You are right,
this is not correct for 32 bit kernel.

I am thinking if I can add a compiling condition check like below. This
only works only when x86_64 or when kaslr is compiled in. Otherwise it
wokrs as before.

#if CONFIG_X86_64
if (output_orig == output)
return;
#endif

or
#if CONFIG_RANDOMIZE_BASE
if (output_orig == output)
return;
#endif

>
> -Kees
>