On Mon, Apr 13, 2020 at 10:24:06PM -0700, Kees Cook wrote: > On Mon, Apr 13, 2020 at 10:40:25PM -0400, Sasha Levin wrote: > > On Fri, Apr 10, 2020 at 10:09:02AM +0200, gregkh@xxxxxxxxxxxxxxxxxxx wrote: > > > > > > The patch below does not apply to the 4.19-stable tree. > > > If someone wants it applied there, or to any other stable or longterm > > > tree, then please email the backport, including the original git commit > > > id to <stable@xxxxxxxxxxxxxxx>. > > > > > > thanks, > > > > > > greg k-h > > > > > > ------------------ original commit in Linus's tree ------------------ > > > > > > From 1ad53d9fa3f6168ebcf48a50e08b170432da2257 Mon Sep 17 00:00:00 2001 > > > From: Kees Cook <keescook@xxxxxxxxxxxx> > > > Date: Wed, 1 Apr 2020 21:04:23 -0700 > > > Subject: [PATCH] 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 > > > > > > 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> > > > > As previously promised, I've grabbed d5767057c9a7 ("uapi: rename > > ext2_swab() to swab() and share globally in swab.h") so that we'll have > > swab() on 4.19 and 4.14, but it wasn't enough. > > > > There was another conflict with d36a63a943e3 ("kasan, slub: fix more > > conflicts with CONFIG_SLAB_FREELIST_HARDENED") which I've resolved by > > simply doing: > > > > diff --git a/mm/slub.c b/mm/slub.c > > index 958a8f7a3c253..d2db6bc5e788b 100644 > > --- a/mm/slub.c > > +++ b/mm/slub.c > > @@ -248,7 +248,7 @@ 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 *)swab((unsigned long)ptr ^ s->random ^ ptr_addr); > > Eeek, no, no. The swab() must be on ptr_addr. I already sent a backport > for this to stable, see: > https://lore.kernel.org/stable/202004131001.20346EB0E7@keescook > > Please use that instead. I've dropped Sasha's backport and grabbed yours instead, thanks. greg k-h
