From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: [patch 024/155] slub: improve bit diffusion for freelist ptr obfuscation Date: Wed, 01 Apr 2020 21:04:23 -0700 Message-ID: <20200402040423.oMxgUNpgm%akpm@linux-foundation.org> References: <20200401210155.09e3b9742e1c6e732f5a7250@linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Return-path: Received: from mail.kernel.org ([198.145.29.99]:49728 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726136AbgDBEEY (ORCPT ); Thu, 2 Apr 2020 00:04:24 -0400 In-Reply-To: <20200401210155.09e3b9742e1c6e732f5a7250@linux-foundation.org> Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: akpm@linux-foundation.org, cl@linux.com, iamjoonsoo.kim@lge.com, keescook@chromium.org, linux-mm@kvack.org, mm-commits@vger.kernel.org, penberg@kernel.org, rientjes@google.com, silvio.cesare@gmail.com, stable@vger.kernel.org, torvalds@linux-foundation.org From: Kees Cook 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 Signed-off-by: Kees Cook Cc: Christoph Lameter Cc: Pekka Enberg Cc: David Rientjes Cc: Joonsoo Kim Cc: Signed-off-by: Andrew Morton --- 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 _