[kernel-hardening] Re: [PATCH v7 0/9] x86/mm: memory area address KASLR

[Kees Cook <keescook@chromium.org>]

On Tue, Jun 21, 2016 at 8:46 PM, Kees Cook <keescook@chromium.org> wrote:
> This is v7 of Thomas Garnier's KASLR for memory areas (physical memory
> mapping, vmalloc, vmemmap). It expects to be applied on top of the
> x86/boot tip.
>
> The current implementation of KASLR randomizes only the base address of
> the kernel and its modules. Research was published showing that static
> memory addresses can be found and used in exploits, effectively ignoring
> base address KASLR:
>
>    The physical memory mapping holds most allocations from boot and
>    heap allocators. Knowning the base address and physical memory
>    size, an attacker can deduce the PDE virtual address for the vDSO
>    memory page.  This attack was demonstrated at CanSecWest 2016, in
>    the "Getting Physical: Extreme Abuse of Intel Based Paged Systems"
>    https://goo.gl/ANpWdV (see second part of the presentation). The
>    exploits used against Linux worked successfuly against 4.6+ but fail
>    with KASLR memory enabled (https://goo.gl/iTtXMJ). Similar research
>    was done at Google leading to this patch proposal. Variants exists
>    to overwrite /proc or /sys objects ACLs leading to elevation of
>    privileges.  These variants were tested against 4.6+.
>
> This set of patches randomizes the base address and padding of three
> major memory sections (physical memory mapping, vmalloc, and vmemmap).
> It mitigates exploits relying on predictable kernel addresses in these
> areas. This feature can be enabled with the CONFIG_RANDOMIZE_MEMORY
> option. (This CONFIG, along with CONFIG_RANDOMIZE may be renamed in
> the future, but stands for now as other architectures continue to
> implement KASLR.)
>
> Padding for the memory hotplug support is managed by
> CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING. The default value is 10
> terabytes.
>
> The patches were tested on qemu & physical machines. Xen compatibility was
> also verified. Multiple reboots were used to verify entropy for each
> memory section.
>
> Notable problems that needed solving:
>  - The three target memory sections need to not be at the same place
>    across reboots.
>  - The physical memory mapping can use a virtual address not aligned on
>    the PGD page table.
>  - Reasonable entropy is needed early at boot before get_random_bytes()
>    is available.
>  - Memory hotplug needs KASLR padding.
>
> Patches:
>  - 1: refactor KASLR functions (moves them from boot/compressed/ into lib/)
>  - 2: clarifies the variables used for physical mapping.
>  - 3: PUD virtual address support for physical mapping.
>  - 4: split out the trampoline PGD
>  - 5: KASLR memory infrastructure code
>  - 6: randomize base of physical mapping region
>  - 7: randomize base of vmalloc region
>  - 8: randomize base of vmemmap region
>  - 9: provide memory hotplug padding support
>
> There is no measurable performance impact:
>
>  - Kernbench shows almost no difference (-+ less than 1%).
>  - Hackbench shows 0% difference on average (hackbench 90 repeated 10 times).

Hi again,

Just a friendly ping -- I'd love to get this into -tip for wider testing.

Thanks!

-Kees


-- 
Kees Cook
Chrome OS & Brillo Security