From: Kees Cook <keescook@xxxxxxxxxxxx> Subject: 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@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 Link: http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook Fixes: 2482ddec670f ("mm: add SLUB free list pointer obfuscation") Reported-by: Silvio Cesare <silvio.cesare@xxxxxxxxx> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Christoph Lameter <cl@xxxxxxxxx> Cc: Pekka Enberg <penberg@xxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/slub.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/mm/slub.c~slub-improve-bit-diffusion-for-freelist-ptr-obfuscation +++ a/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 _
