[PATCH 2/2] mm/page_alloc.c: add config option to sanitize freed pages

From: Anisse Astier
Date: Fri Apr 24 2015 - 17:06:41 EST

This new config option will sanitize all freed pages. This is a pretty
low-level change useful to track some cases of use-after-free, help
kernel same-page merging in VM environments, and counter a few info

Signed-off-by: Anisse Astier <anisse@xxxxxxxxx>
mm/Kconfig | 12 ++++++++++++
mm/page_alloc.c | 5 +++++
2 files changed, 17 insertions(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index 390214d..cb2df5f 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -635,3 +635,15 @@ config MAX_STACK_SIZE_MB
changed to a smaller value in which case that is used.

A sane initial value is 80 MB.
+ bool "Sanitize memory pages after free"
+ default n
+ help
+ This option is used to make sure all pages freed are zeroed. This is
+ quite low-level and doesn't handle your slab buffers.
+ It has various applications, from preventing some info leaks to
+ helping kernel same-page merging in virtualised environments.
+ Depending on your workload, it will reduce performance of about 3%.
+ If unsure, say N.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 05fcec9..c71440a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -803,6 +803,11 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
PAGE_SIZE << order);
+ zero_pages(page, order);
arch_free_page(page, order);
kernel_map_pages(page, 1 << order, 0);


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