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Message-Id: <20170705163957.90c7856f622a63666df4b5a6@linux-foundation.org>
Date: Wed, 5 Jul 2017 16:39:57 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Kees Cook <keescook@...omium.org>
Cc: Christoph Lameter <cl@...ux.com>, Laura Abbott <labbott@...hat.com>,
Daniel Micay <danielmicay@...il.com>, Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Ingo Molnar
<mingo@...nel.org>, Josh Triplett <josh@...htriplett.org>, Andy Lutomirski
<luto@...nel.org>, Nicolas Pitre <nicolas.pitre@...aro.org>, Tejun Heo
<tj@...nel.org>, Daniel Mack <daniel@...que.org>, Sebastian Andrzej Siewior
<bigeasy@...utronix.de>, Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Helge Deller <deller@....de>, Rik van Riel <riel@...hat.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2] mm: Add SLUB free list pointer obfuscation
On Thu, 22 Jun 2017 18:50:10 -0700 Kees Cook <keescook@...omium.org> wrote:
> This SLUB free list pointer obfuscation code is modified from Brad
> Spengler/PaX Team's code in the last public patch of grsecurity/PaX based
> on my understanding of the code. Changes or omissions from the original
> code are mine and don't reflect the original grsecurity/PaX code.
>
> This adds a per-cache random value to SLUB caches that is XORed with
> their freelist pointers. This adds nearly zero overhead and frustrates the
> very common heap overflow exploitation method of overwriting freelist
> pointers. A recent example of the attack is written up here:
> http://cyseclabs.com/blog/cve-2016-6187-heap-off-by-one-exploit
>
> This is based on patches by Daniel Micay, and refactored to avoid lots
> of #ifdef code.
>
> ...
>
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1900,6 +1900,15 @@ config SLAB_FREELIST_RANDOM
> security feature reduces the predictability of the kernel slab
> allocator against heap overflows.
>
> +config SLAB_FREELIST_HARDENED
> + bool "Harden slab freelist metadata"
> + depends on SLUB
> + help
> + Many kernel heap attacks try to target slab cache metadata and
> + other infrastructure. This options makes minor performance
> + sacrifies to harden the kernel slab allocator against common
> + freelist exploit methods.
> +
Well, it is optable-outable.
> config SLUB_CPU_PARTIAL
> default y
> depends on SLUB && SMP
> diff --git a/mm/slub.c b/mm/slub.c
> index 57e5156f02be..590e7830aaed 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -34,6 +34,7 @@
> #include <linux/stacktrace.h>
> #include <linux/prefetch.h>
> #include <linux/memcontrol.h>
> +#include <linux/random.h>
>
> #include <trace/events/kmem.h>
>
> @@ -238,30 +239,50 @@ static inline void stat(const struct kmem_cache *s, enum stat_item si)
> * Core slab cache functions
> *******************************************************************/
>
> +#ifdef CONFIG_SLAB_FREELIST_HARDENED
> +# define initialize_random(s) \
> + do { \
> + s->random = get_random_long(); \
> + } while (0)
> +# define FREEPTR_VAL(ptr, ptr_addr, s) \
> + (void *)((unsigned long)(ptr) ^ s->random ^ (ptr_addr))
> +#else
> +# define initialize_random(s) do { } while (0)
> +# define FREEPTR_VAL(ptr, addr, s) ((void *)(ptr))
> +#endif
> +#define FREELIST_ENTRY(ptr_addr, s) \
> + FREEPTR_VAL(*(unsigned long *)(ptr_addr), \
> + (unsigned long)ptr_addr, s)
> +
That's a bit of an eyesore. Is there any reason why we cannot
implement all of the above in nice, conventional C functions?
>
> ...
>
> @@ -3536,6 +3557,7 @@ static int kmem_cache_open(struct kmem_cache *s, unsigned long flags)
> {
> s->flags = kmem_cache_flags(s->size, flags, s->name, s->ctor);
> s->reserved = 0;
> + initialize_random(s);
>
> if (need_reserve_slab_rcu && (s->flags & SLAB_TYPESAFE_BY_RCU))
> s->reserved = sizeof(struct rcu_head);
We regularly have issues where the random system just isn't ready
(enough) for clients to use it. Are you sure the above is actually
useful for the boot-time caches?
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