[PATCH v7 0/9] x86/mm: memory area address KASLR

[Kees Cook]

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).