From: Kees Cook keescook@chromium.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@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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- mm/slub.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/slub.c +++ b/mm/slub.c @@ -248,7 +248,7 @@ static inline void *freelist_ptr(const s 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