[kernel-hardening] Re: [PATCH v8 1/4] x86/KASLR: Clarify identity map interface

Kees Cook
2016-05-11T15:23:45Z

On Tue, May 10, 2016 at 11:24 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Kees Cook <keescook@chromium.org> wrote:
>
>> +/*
>> + * Mapping information structure passed to kernel_ident_mapping_init().
>> + * Due to relocation, pointers must be assigned at run time not build time.
>> + */
>> +static struct x86_mapping_info mapping_info = {
>> + .pmd_flag = __PAGE_KERNEL_LARGE_EXEC,
>> +};
>
>> +void initialize_identity_maps(void)
>> {
>> + /* Init mapping_info with run-time function/buffer pointers. */
>> + mapping_info.alloc_pgt_page = alloc_pgt_page;
>> + mapping_info.context = &pgt_data;
>
> Could you please outline the precise failure mode? What gets executed when, which
> pointer gets relocated and which not, and exactly when does it pose a problem,
> etc.

It's the issue described at the top of misc.c:

/*
* WARNING!!
* This code is compiled with -fPIC and it is relocated dynamically at
* run time, but no relocation processing is performed. This means that
* it is not safe to place pointers in static structures.
*/

Should this be repeated in each .c file maybe as a reminder?

If the rest of the patches look good, we could clean this up as a
following patch? What do you think?

-Kees

--
Kees Cook
Chrome OS & Brillo Security