| From 606c4677696091ccb32267b3ad3971b6233979e6 Mon Sep 17 00:00:00 2001 |
| From: Kees Cook <keescook@chromium.org> |
| 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) |
| ... |
| |
| after: |
| |
| 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) |
| |
| [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@gmail.com> |
| Signed-off-by: Kees Cook <keescook@chromium.org> |
| Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
| Cc: Christoph Lameter <cl@linux.com> |
| Cc: Pekka Enberg <penberg@kernel.org> |
| Cc: David Rientjes <rientjes@google.com> |
| Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> |
| Cc: <stable@vger.kernel.org> |
| Link: http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook |
| Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
| Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com> |
| |
| 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))); |
| #else |
| return ptr; |
| #endif |
| -- |
| 2.7.4 |
| |