Re: [PATCH v4 19/20] x86, kaslr: Allow random address to be below loaded address

[Baoquan He <bhe@redhat.com>]

On 03/22/16 at 12:54pm, Kees Cook wrote:
> On Tue, Mar 22, 2016 at 12:32 AM, Baoquan He <bhe@redhat.com> wrote:
> > From: Yinghai Lu <yinghai@kernel.org>
> >
> > Now new randomized output can only be chosen from regions above loaded
> > address. In this case, for bootloaders like kexec which always loads
> > kernel near the end of ram, it doesn't do randomization at all. Or kernel
> > is loaded in a very big starting address, we should not give up that area
> > is loaded in a very large address, then the area below the large loaded
> > address will be given up. This is not reasonable.
> >
> > With correct tracking in mem_avoid  we can allow random output below
> > loaded address. With this change, though kexec can get random ouput
> > below its loaded address of kernel.
> >
> > Now we just pick 512M as min_addr. If kernel loaded address is bigger than
> > 512M, E.g 8G. Then [512M, 8G) can be added into random output candidate area.
> >
> > Signed-off-by: Yinghai Lu <yinghai@kernel.org>
> > ---
> >  arch/x86/boot/compressed/aslr.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
> > index ddfc3d0..d072ca7 100644
> > --- a/arch/x86/boot/compressed/aslr.c
> > +++ b/arch/x86/boot/compressed/aslr.c
> > @@ -446,7 +446,8 @@ void choose_kernel_location(unsigned char *input,
> >                                 unsigned long output_size,
> >                                 unsigned char **virt_offset)
> >  {
> > -       unsigned long random;
> > +       unsigned long random, min_addr;
> > +
> >         *virt_offset = (unsigned char *)LOAD_PHYSICAL_ADDR;
> >
> >  #ifdef CONFIG_HIBERNATION
> > @@ -467,8 +468,13 @@ void choose_kernel_location(unsigned char *input,
> >         mem_avoid_init((unsigned long)input, input_size,
> >                        (unsigned long)*output);
> >
> > +       /* start from 512M */
> > +       min_addr = (unsigned long)*output;
> > +       if (min_addr > (512UL<<20))
> > +               min_addr = 512UL<<20;
> 
> The goal is to find a minimum address? I'm not sure this comment makes
> sense. Shouldn't this be:
> 
>     /* Lower minimum to 512M. */
>    min_addr = min_t(unsigned long, *output, 512UL << 20);
> 
> Or something like that?

Yes, the goal is to lower minimum to 512M. It's better to change it to
"Lower minimum to 512M" as you suggested. I will resend a new one with
this update into this thread.

Thanks a lot for your great suggestion for patch log, code change and
patch rearranging, and also appreciate your encouragement and patience.

Thanks
Baoquan

> 
> > +
> >         /* Walk e820 and find a random address. */
> > -       random = find_random_phy_addr((unsigned long)*output, output_size);
> > +       random = find_random_phy_addr(min_addr, output_size);
> >         if (!random)
> >                 debug_putstr("KASLR could not find suitable E820 region...\n");
> >         else {
> > --
> > 2.5.0
> >
> 
> -Kees
> 
> -- 
> Kees Cook
> Chrome OS & Brillo Security