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From: Kees Cook <>
To: Andrew Morton <>
Cc: Christoph Lameter <>,
	Pekka Enberg <>,
	David Rientjes <>,
	Joonsoo Kim <>,
	Daniel Micay <>,
	Silvio Cesare <>,,
Subject: [PATCH] slub: Improve bit diffusion for freelist ptr obfuscation
Date: Thu, 5 Mar 2020 16:24:28 -0800	[thread overview]
Message-ID: <202003051623.AF4F8CB@keescook> (raw)

Under CONFIG_SLAB_FREELIST_HARDENED=y, the obfuscation was relatively weak
in that the ptr and ptr address were usually so close that the first XOR
would result in an almost entirely 0-byte value[1], leaving most of the
"secret" number ultimately being stored after the third XOR. A single
blind memory content exposure of the freelist was generally sufficient
to learn the secret.

Add a swab() call to mix bits a little more. This is a cheap way
(1 cycle) to make attacks need more than a single exposure to learn
the secret (or to know _where_ the exposure is in memory).

kmalloc-32 freelist walk, before:

ptr              ptr_addr            stored value      secret
ffff90c22e019020@ffff90c22e019000 is 86528eb656b3b5bd (86528eb656b3b59d)
ffff90c22e019040@ffff90c22e019020 is 86528eb656b3b5fd (86528eb656b3b59d)
ffff90c22e019060@ffff90c22e019040 is 86528eb656b3b5bd (86528eb656b3b59d)
ffff90c22e019080@ffff90c22e019060 is 86528eb656b3b57d (86528eb656b3b59d)
ffff90c22e0190a0@ffff90c22e019080 is 86528eb656b3b5bd (86528eb656b3b59d)


ptr              ptr_addr            stored value      secret
ffff9eed6e019020@ffff9eed6e019000 is 793d1135d52cda42 (86528eb656b3b59d)
ffff9eed6e019040@ffff9eed6e019020 is 593d1135d52cda22 (86528eb656b3b59d)
ffff9eed6e019060@ffff9eed6e019040 is 393d1135d52cda02 (86528eb656b3b59d)
ffff9eed6e019080@ffff9eed6e019060 is 193d1135d52cdae2 (86528eb656b3b59d)
ffff9eed6e0190a0@ffff9eed6e019080 is f93d1135d52cdac2 (86528eb656b3b59d)


Reported-by: Silvio Cesare <>
Fixes: 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
Signed-off-by: Kees Cook <>
 mm/slub.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 17dc00e33115..107d9d89cf96 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -259,7 +259,7 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
 	 * freepointer to be restored incorrectly.
 	return (void *)((unsigned long)ptr ^ s->random ^
-			(unsigned long)kasan_reset_tag((void *)ptr_addr));
+			swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
 	return ptr;

Kees Cook

                 reply	other threads:[~2020-03-06  0:24 UTC|newest]

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