Re: FAILED: patch "[PATCH] slub: improve bit diffusion for freelist ptr obfuscation" failed to apply to 4.19-stable tree

[Kees Cook <keescook@chromium.org>]

On Mon, Apr 13, 2020 at 10:40:25PM -0400, Sasha Levin wrote:
> On Fri, Apr 10, 2020 at 10:09:02AM +0200, gregkh@linuxfoundation.org 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@vger.kernel.org>.
> > 
> > thanks,
> > 
> > greg k-h
> > 
> > ------------------ original commit in Linus's tree ------------------
> > 
> > From 1ad53d9fa3f6168ebcf48a50e08b170432da2257 Mon Sep 17 00:00:00 2001
> > From: Kees Cook <keescook@chromium.org>
> > 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@gmail.com>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Pekka Enberg <penberg@kernel.org>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: <stable@vger.kernel.org>
> > Link: http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook
> > Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> 
> 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.

-- 
Kees Cook