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

[Kees Cook <keescook@chromium.org>]

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?

> +
>         /* 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