[RFC v1 0/4] x86, boot: KASLR memory implementation (x86_64)

Thomas Garnier
2016-04-15T22:04:34Z

This is RFC v1 for KASLR memory implementation on x86_64. It was reviewed
early by Kees Cook.

***Background:
The current implementation of KASLR randomizes only the base address of
the kernel and its modules. Research was published showing that static
memory can be overwitten to elevate privileges bypassing KASLR.

In more details:

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). 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.

This set of patches randomizes base address and padding of three
major memory sections (physical memory mapping, vmalloc & vmemmap).
It mitigates exploits relying on predictable kernel addresses. This
feature can be enabled with the CONFIG_RANDOMIZE_MEMORY option.

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.

***Problems that needed solving:
- The three target memory sections are never at the same place between
boots.
- The physical memory mapping can use a virtual address not aligned on
the PGD page table.
- Have good entropy early at boot before get_random_bytes is available.
- Add optional padding for memory hotplug compatibility.

***Parts:
- The first part prepares for the KASLR memory randomization by
refactoring entropy functions used by the current implementation and
support PUD level virtual addresses for physical mapping.
(Patches 01-02)
- The second part implements the KASLR memory randomization for all
sections mentioned.
(Patch 03)
- The third part adds support for memory hotplug by adding an option to
define the padding used between the physical memory mapping section
and the others.
(Patch 04)

Thanks!

Thomas