linux-kernel - Re: [PATCH] x86, boot: skip relocs when load address unchanged

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Date:	Thu, 26 Feb 2015 07:29:30 +0100
From:	MegaBrutal <megabrutal@...il.com>
To:	Baoquan He <bhe@...hat.com>
Cc:	Kees Cook <keescook@...omium.org>,
	Linux kernel <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>, Vivek Goyal <vgoyal@...hat.com>,
	Jan Beulich <JBeulich@...e.com>,
	Junjie Mao <eternal.n08@...il.com>,
	Andi Kleen <ak@...ux.intel.com>, "Thomas D." <whissi@...ssi.de>
Subject: Re: [PATCH] x86, boot: skip relocs when load address unchanged

Thanks for this patch, and good to see it in mainline!

This actually fixes the problem I reported here:
https://lkml.org/lkml/2014/12/1/15

I wish it to be backported into the Ubuntu Utopic kernel asap.

> This patch works for me. And good to see it's being merged. About the
> patch log, I would say relocations do unexpected things when the kernel
> is above 1G since randomization is done from 16M to 1G, namely
> CONFIG_RANDOMIZE_BASE_MAX_OFFSET. So above 1G kernel text mapping will
> step into kernel module mapping region.

I'm just speculating, but is it the reason why I only get problems
when I boot with kexec? Maybe it's only then when the kernel gets
above 1G. Otherwise, the kernels boot properly when they are booted
from GRUB.


2015-01-20 14:04 GMT+01:00 Baoquan He <bhe@...hat.com>:
>
> On 01/15/15 at 04:51pm, Kees Cook wrote:
> > On 64-bit, relocation is not required unless the load address gets
> > changed. Without this, relocations do unexpected things when the kernel
> > is above 4G.
>
> This patch works for me. And good to see it's being merged. About the
> patch log, I would say relocations do unexpected things when the kernel
> is above 1G since randomization is done from 16M to 1G, namely
> CONFIG_RANDOMIZE_BASE_MAX_OFFSET. So above 1G kernel text mapping will
> step into kernel module mapping region.
>
> BTW, I am working on separate randomization of kernel physical and virtual
> address , will post it. But it won't conflict with this because I don't
> think it can be accepted in a short time. Before that this patch truly
> fix the kexec/kdump bug when kaslr is compiled in.
>
> Thanks
> Baoquan
>
> >
> > Reported-by: Baoquan He <bhe@...hat.com>
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > Cc: stable@...r.kernel.org
> > ---
> > This is a reimplementation of Baoquan's "kaslr: check if kernel location is
> > changed", which performs the check without needing to change the function
> > declaration. This should have exactly the same effect, but I dropped Vivek's
> > Ack and Thomas's Test, since it's technically a different patch.
> > ---
> >  arch/x86/boot/compressed/misc.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> > index dcc1c536cc21..a950864a64da 100644
> > --- a/arch/x86/boot/compressed/misc.c
> > +++ b/arch/x86/boot/compressed/misc.c
> > @@ -373,6 +373,8 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
> >                                 unsigned long output_len,
> >                                 unsigned long run_size)
> >  {
> > +     unsigned char *output_orig = output;
> > +
> >       real_mode = rmode;
> >
> >       sanitize_boot_params(real_mode);
> > @@ -421,7 +423,12 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
> >       debug_putstr("\nDecompressing Linux... ");
> >       decompress(input_data, input_len, NULL, NULL, output, NULL, error);
> >       parse_elf(output);
> > -     handle_relocations(output, output_len);
> > +     /*
> > +      * 32-bit always performs relocations. 64-bit relocations are only
> > +      * needed if kASLR has chosen a different load address.
> > +      */
> > +     if (!IS_ENABLED(CONFIG_X86_64) || output != output_orig)
> > +             handle_relocations(output, output_len);
> >       debug_putstr("done.\nBooting the kernel.\n");
> >       return output;
> >  }
> > --
> > 1.9.1
> >
> >
> > --
> > Kees Cook
> > Chrome OS Security
> --
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