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Date: Sat, 11 Apr 2020 14:09:40 +0200 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: linux-kernel@...r.kernel.org Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org, Silvio Cesare <silvio.cesare@...il.com>, Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: [PATCH 5.5 20/44] slub: improve bit diffusion for freelist ptr obfuscation From: Kees Cook <keescook@...omium.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@...f90c22e019000 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019040@...f90c22e019020 is 86528eb656b3b5fd (86528eb656b3b59d) ffff90c22e019060@...f90c22e019040 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019080@...f90c22e019060 is 86528eb656b3b57d (86528eb656b3b59d) ffff90c22e0190a0@...f90c22e019080 is 86528eb656b3b5bd (86528eb656b3b59d) ... after: ptr ptr_addr stored value secret ffff9eed6e019020@...f9eed6e019000 is 793d1135d52cda42 (86528eb656b3b59d) ffff9eed6e019040@...f9eed6e019020 is 593d1135d52cda22 (86528eb656b3b59d) ffff9eed6e019060@...f9eed6e019040 is 393d1135d52cda02 (86528eb656b3b59d) ffff9eed6e019080@...f9eed6e019060 is 193d1135d52cdae2 (86528eb656b3b59d) ffff9eed6e0190a0@...f9eed6e019080 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@...il.com> Signed-off-by: Kees Cook <keescook@...omium.org> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org> Cc: Christoph Lameter <cl@...ux.com> Cc: Pekka Enberg <penberg@...nel.org> Cc: David Rientjes <rientjes@...gle.com> Cc: Joonsoo Kim <iamjoonsoo.kim@....com> Cc: <stable@...r.kernel.org> Link: http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org> --- mm/slub.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/mm/slub.c +++ b/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
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