linux-kernel - [PATCH 5.5 20/44] slub: improve bit diffusion for freelist ptr obfuscation

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Message-Id: <20200411115458.744487200@linuxfoundation.org>
Date:   Sat, 11 Apr 2020 14:09:40 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Silvio Cesare <silvio.cesare@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 5.5 20/44] slub: improve bit diffusion for freelist ptr obfuscation

From: Kees Cook <keescook@...omium.org>

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@...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

Fixes: 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
Reported-by: Silvio Cesare <silvio.cesare@...il.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Pekka Enberg <penberg@...nel.org>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
Cc: <stable@...r.kernel.org>
Link: http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/slub.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/slub.c
+++ b/mm/slub.c
@@ -259,7 +259,7 @@ static inline void *freelist_ptr(const s
 	 * 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