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

[Anisse Astier]

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
leaks.

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.
+
+config SANITIZE_FREED_PAGES
+	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)
 		debug_check_no_obj_freed(page_address(page),
 					   PAGE_SIZE << order);
 	}
+
+#ifdef CONFIG_SANITIZE_FREED_PAGES
+	zero_pages(page, order);
+#endif
+
 	arch_free_page(page, order);
 	kernel_map_pages(page, 1 << order, 0);
 
-- 
1.9.3

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