On Mon, Apr 13, 2020 at 10:03:23AM -0700, Kees Cook wrote:

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 [kees: Backport to v4.19 which doesn't call kasan_reset_untag()] Signed-off-by: Kees Cook keescook@chromium.org

---

This requires that commit d5767057c9a76a29f073dad66b7fa12a90e8c748 is also cherry-picked into -stable.

---

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

diff --git a/mm/slub.c b/mm/slub.c index 9b7b989273d4..d8116a43a287 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -249,7 +249,7 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr, unsigned long ptr_addr) { #ifdef CONFIG_SLAB_FREELIST_HARDENED

• return (void *)((unsigned long)ptr ^ s->random ^ ptr_addr);

• return (void *)((unsigned long)ptr ^ s->random ^ swab(ptr_addr));

#else return ptr;

#endif

2.20.1

--

Now queued up, thanks.

greg k-h