linux-kernel - [PATCH] slub: Improve bit diffusion for freelist ptr obfuscation

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Date:   Thu, 5 Mar 2020 16:24:28 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Daniel Micay <danielmicay@...il.com>,
        Silvio Cesare <silvio.cesare@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] slub: Improve bit diffusion for freelist ptr obfuscation

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@...f90c22e019000 is 86528eb656b3b5bd (86528eb656b3b59d)
ffff90c22e019040@...f90c22e019020 is 86528eb656b3b5fd (86528eb656b3b59d)
ffff90c22e019060@...f90c22e019040 is 86528eb656b3b5bd (86528eb656b3b59d)
ffff90c22e019080@...f90c22e019060 is 86528eb656b3b57d (86528eb656b3b59d)
ffff90c22e0190a0@...f90c22e019080 is 86528eb656b3b5bd (86528eb656b3b59d)
...

after:

ptr              ptr_addr            stored value      secret
ffff9eed6e019020@...f9eed6e019000 is 793d1135d52cda42 (86528eb656b3b59d)
ffff9eed6e019040@...f9eed6e019020 is 593d1135d52cda22 (86528eb656b3b59d)
ffff9eed6e019060@...f9eed6e019040 is 393d1135d52cda02 (86528eb656b3b59d)
ffff9eed6e019080@...f9eed6e019060 is 193d1135d52cdae2 (86528eb656b3b59d)
ffff9eed6e0190a0@...f9eed6e019080 is f93d1135d52cdac2 (86528eb656b3b59d)

[1] https://blog.infosectcbr.com.au/2020/03/weaknesses-in-linux-kernel-heap.html

Reported-by: Silvio Cesare <silvio.cesare@...il.com>
Fixes: 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
Cc: stable@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 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)));
 #else
 	return ptr;
 #endif
-- 
2.20.1


-- 
Kees Cook