linux-kernel - [PATCH v4 2/3] mm/page_alloc.c: add config option to sanitize freed pages

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Date:	Thu, 14 May 2015 16:19:47 +0200
From:	Anisse Astier <anisse@...ier.eu>
To:	unlisted-recipients:; (no To-header on input)
Cc:	Anisse Astier <anisse@...ier.eu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	David Rientjes <rientjes@...gle.com>,
	Alan Cox <gnomes@...rguk.ukuu.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	PaX Team <pageexec@...email.hu>,
	Brad Spengler <spender@...ecurity.net>,
	Kees Cook <keescook@...omium.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	linux-mm@...ck.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v4 2/3] mm/page_alloc.c: add config option to sanitize freed pages

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@...ier.eu>
---
 mm/Kconfig      | 12 ++++++++++++
 mm/page_alloc.c | 12 ++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index 390214d..e9fb3bd 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 greatly reduce performance.
+
+	  If unsure, say N.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4d5ce6e..c29e3a0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -795,6 +795,12 @@ 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
+	for (i = 0; i < (1 << order); i++)
+		clear_highpage(page + i);
+#endif
+
 	arch_free_page(page, order);
 	kernel_map_pages(page, 1 << order, 0);
 
@@ -960,9 +966,15 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
 	kernel_map_pages(page, 1 << order, 1);
 	kasan_alloc_pages(page, order);
 
+#ifndef CONFIG_SANITIZE_FREED_PAGES
+	/* SANITIZE_FREED_PAGES relies implicitly on the fact that pages are
+	 * cleared before use, so we don't need gfp zero in the default case
+	 * because all pages go through the free_pages_prepare code path when
+	 * switching from bootmem to the default allocator */
 	if (gfp_flags & __GFP_ZERO)
 		for (i = 0; i < (1 << order); i++)
 			clear_highpage(page + i);
+#endif
 
 	if (order && (gfp_flags & __GFP_COMP))
 		prep_compound_page(page, order);
-- 
1.9.3

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