Re: [RFC PATCH v2 0/3] Add support for eXclusive Page Frame Ownership (XPFO)

[Juerg Haefliger]

Resending to include the kernel-hardening list. Sorry, I wasn't subscribed with the correct email
address when I sent this the first time.

...Juerg

On 09/14/2016 09:18 AM, Juerg Haefliger wrote:
> Changes from:
>   v1 -> v2:
>     - Moved the code from arch/x86/mm/ to mm/ since it's (mostly)
>       arch-agnostic.
>     - Moved the config to the generic layer and added ARCH_SUPPORTS_XPFO
>       for x86.
>     - Use page_ext for the additional per-page data.
>     - Removed the clearing of pages. This can be accomplished by using
>       PAGE_POISONING.
>     - Split up the patch into multiple patches.
>     - Fixed additional issues identified by reviewers.
> 
> This patch series adds support for XPFO which protects against 'ret2dir'
> kernel attacks. The basic idea is to enforce exclusive ownership of page
> frames by either the kernel or userspace, unless explicitly requested by
> the kernel. Whenever a page destined for userspace is allocated, it is
> unmapped from physmap (the kernel's page table). When such a page is
> reclaimed from userspace, it is mapped back to physmap.
> 
> Additional fields in the page_ext struct are used for XPFO housekeeping.
> Specifically two flags to distinguish user vs. kernel pages and to tag
> unmapped pages and a reference counter to balance kmap/kunmap operations
> and a lock to serialize access to the XPFO fields.
> 
> Known issues/limitations:
>   - Only supports x86-64 (for now)
>   - Only supports 4k pages (for now)
>   - There are most likely some legitimate uses cases where the kernel needs
>     to access userspace which need to be made XPFO-aware
>   - Performance penalty
> 
> Reference paper by the original patch authors:
>   http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf
> 
> Juerg Haefliger (3):
>   Add support for eXclusive Page Frame Ownership (XPFO)
>   xpfo: Only put previous userspace pages into the hot cache
>   block: Always use a bounce buffer when XPFO is enabled
> 
>  arch/x86/Kconfig         |   3 +-
>  arch/x86/mm/init.c       |   2 +-
>  block/blk-map.c          |   2 +-
>  include/linux/highmem.h  |  15 +++-
>  include/linux/page_ext.h |   7 ++
>  include/linux/xpfo.h     |  41 +++++++++
>  lib/swiotlb.c            |   3 +-
>  mm/Makefile              |   1 +
>  mm/page_alloc.c          |  10 ++-
>  mm/page_ext.c            |   4 +
>  mm/xpfo.c                | 213 +++++++++++++++++++++++++++++++++++++++++++++++
>  security/Kconfig         |  20 +++++
>  12 files changed, 314 insertions(+), 7 deletions(-)
>  create mode 100644 include/linux/xpfo.h
>  create mode 100644 mm/xpfo.c
> 


-- 
Juerg Haefliger
Hewlett Packard Enterprise

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