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

[Kees Cook]

On Fri, Jun 17, 2016 at 1:47 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> From: Yinghai Lu <yinghai@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> [kees: rewrote changelog, refactor to use min()]
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>>  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