On Fri, Jun 17, 2016 at 1:47 AM, Ingo Molnar <mi...@kernel.org> wrote:
>
> * Kees Cook <keesc...@chromium.org> wrote:
>
>> From: Yinghai Lu <ying...@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 <ying...@kernel.org>
>> [kees: rewrote changelog, refactor to use min()]
>> Signed-off-by: Kees Cook <keesc...@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.
Yinghai, do you have a rationale for this selection? I understood it
to just be a very conservative target to avoid anything in low
physical memory, but perhaps there is a better reason?
-Kees
--
Kees Cook
Chrome OS & Brillo Security
