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

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Date:	Wed, 10 Sep 2014 09:20:21 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Baoquan He <bhe@...hat.com>
Cc:	Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Deutschmann <whissi@...ssi.de>,
	Dave Young <dyoung@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	WANG Chao <chaowang@...hat.com>
Subject: Re: [PATCH 1/4] kaslr: check user's config too when handle
 relocations

On Wed, Sep 10, 2014 at 02:10:35PM +0800, Baoquan He wrote:

[..]
> > > 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

Hi Bao,

I think above should work reasonably well. Also put a comment above it.
Something like as follows.

/*
 * 32bit always requires relocations to be performed. For x86_64,
 * relocations need to be performed only if kaslr has chosen a
 * different load address then kernel was originally loaded at.
 *
 * If we are here, either kaslr is not configured in or kaslr is disabled
 * or kaslr has chosen not to change the load location of kernel. Don't
 * perform any relocations.
 */

Thanks
Vivek
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