Re: [PATCH v9 5/5] x86/KASLR: Allow randomization below load address

[Ingo Molnar <mingo@kernel.org>]

* Kees Cook <keescook@chromium.org> wrote:

> From: Yinghai Lu <yinghai@kernel.org>
> 
> Currently the physical randomization's lower boundary is the original
> kernel load address. For bootloaders that load kernels into very high
> memory (e.g. kexec), this means randomization takes place in a very small
> window at the top of memory, ignoring the large region of physical memory
> below the load address.
> 
> Since mem_avoid is already correctly tracking the regions that must be
> avoided, this patch changes the minimum address to whatever is less:
> 512M (to conservatively avoid unknown things in lower memory) or the
> load address. Now, for example, if the kernel is loaded at 8G, [512M,
> 8G) will be added into possible physical memory positions.
> 
> Signed-off-by: Yinghai Lu <yinghai@kernel.org>
> [kees: rewrote changelog, refactor to use min()]
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/x86/boot/compressed/kaslr.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index d0a823df183b..304c5c369aff 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -492,7 +492,7 @@ void choose_random_location(unsigned long input,
>  			    unsigned long output_size,
>  			    unsigned long *virt_addr)
>  {
> -	unsigned long random_addr;
> +	unsigned long random_addr, min_addr;
>  
>  	/* By default, keep output position unchanged. */
>  	*virt_addr = *output;
> @@ -517,8 +517,11 @@ void choose_random_location(unsigned long input,
>  	/* Record the various known unsafe memory ranges. */
>  	mem_avoid_init(input, input_size, *output);
>  
> +	/* Low end should be the smaller of 512M or initial location. */
> +	min_addr = min(*output, 512UL << 20);
> +
>  	/* Walk e820 and find a random address. */
> -	random_addr = find_random_phys_addr(*output, output_size);
> +	random_addr = find_random_phys_addr(min_addr, output_size);
>  	if (!random_addr) {
>  		warn("KASLR disabled: could not find suitable E820 region!");
>  	} else {

There's no explanation in the code or in the changelog of why 512M was picked as 
the lower limit.

Thanks,

	Ingo