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@xxxxxxxxx> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Christoph Lameter <cl@xxxxxxxxx> > Cc: Pekka Enberg <penberg@xxxxxxxxxx> > Cc: David Rientjes <rientjes@xxxxxxxxxx> > Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> > Link: http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook > Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > [kees: Backport to v4.19 which doesn't call kasan_reset_untag()] > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > --- > 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
