[PATCH v4 0/3] Sanitizing freed pages

[Anisse Astier]

Hi,

I'm trying revive an old debate here[1], though with a simpler approach than
was previously tried. This patch series implements a new option to sanitize
freed pages, a (very) small subset of what is done in PaX/grsecurity[3],
inspired by a previous submission [4].

There are a few different uses that this can cover:
 - some cases of use-after-free could be detected (crashes), although this not
   as efficient as KAsan/kmemcheck
 - it can help with long-term memory consumption in an environment with
   multiple VMs and Kernel Same-page Merging on the host. [2]
 - finally, it can reduce infoleaks, although this is hard to measure.

The approach is voluntarily kept as simple as possible. A single configuration
option, no command line option, no sysctl nob. It can of course be changed,
although I'd be wary of runtime-configuration options that could be used for
races.

I haven't been able to measure a meaningful performance difference when
compiling a (in-cache) kernel; I'd be interested to see what difference it
makes with your particular workload/hardware (I suspect mine is CPU-bound on
this small laptop).

First patch fixes the hibernate use case which will load all the pages of the
restored kernel, and then jump into it, leaving the loader kernel pages hanging
around unclean. We use the free pages bitmap to know which pages should be
cleaned after restore.

Third patch is debug code that can be used to find issues if this feature fails
on your system. It shouldn't necessarily be merged.

Changes since v3:
 - drop original first patch, it has been queued by Andrew in mmotm
 - fix issue raised by Pavel Machek in hibernate patch
 - checkpatch issue in third patch

Changes since v2:
 - reorder patches to fix hibernate first
 - update debug patch to use memchr_inv
 - cc linux-pm and maintainers

Changes since v1:
 - fix some issues raised by David Rientjes, Andi Kleen and PaX Team.
 - add hibernate fix (third patch)
 - add debug code, this is "just in case" someone has an issue with this
   feature. Not sure if it should be merged.


[1] https://lwn.net/Articles/334747/
[2] https://staff.aist.go.jp/k.suzaki/EuroSec12-SUZAKI-revised2.pdf
[3] http://en.wikibooks.org/wiki/Grsecurity/Appendix/Grsecurity_and_PaX_Configuration_Options#Sanitize_all_freed_memory
[4] http://article.gmane.org/gmane.linux.kernel.mm/34398


Anisse Astier (3):
  PM / Hibernate: prepare for SANITIZE_FREED_PAGES
  mm/page_alloc.c: add config option to sanitize freed pages
  mm: Add debug code for SANITIZE_FREED_PAGES

 kernel/power/hibernate.c |  4 +++-
 kernel/power/power.h     |  2 ++
 kernel/power/snapshot.c  | 26 ++++++++++++++++++++++++++
 mm/Kconfig               | 22 ++++++++++++++++++++++
 mm/page_alloc.c          | 30 ++++++++++++++++++++++++++++++
 5 files changed, 83 insertions(+), 1 deletion(-)

-- 
1.9.3

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