blob: d2f3018a358b5f3fdb363f40e04e9e308ea1209b [file] [log] [blame]
From 606c4677696091ccb32267b3ad3971b6233979e6 Mon Sep 17 00:00:00 2001
From: Kees Cook <>
Date: Wed, 1 Apr 2020 21:04:23 -0700
Subject: [PATCH] slub: improve bit diffusion for freelist ptr obfuscation
commit 1ad53d9fa3f6168ebcf48a50e08b170432da2257 upstream.
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)
Fixes: 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
Reported-by: Silvio Cesare <>
Signed-off-by: Kees Cook <>
Signed-off-by: Andrew Morton <>
Cc: Christoph Lameter <>
Cc: Pekka Enberg <>
Cc: David Rientjes <>
Cc: Joonsoo Kim <>
Cc: <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Paul Gortmaker <>
diff --git a/mm/slub.c b/mm/slub.c
index 2179d722a1c3..6fa661d13fd2 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -261,7 +261,7 @@ 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;