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Message-ID: <20160427005505.GA6336@js1304-P5Q-DELUXE>
Date: Wed, 27 Apr 2016 09:55:05 +0900
From: Joonsoo Kim <iamjoonsoo.kim@....com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Thomas Garnier <thgarnie@...gle.com>, Christoph Lameter <cl@...ux.com>,
Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>,
Kees Cook <keescook@...omium.org>, gthelen@...gle.com,
labbott@...oraproject.org, kernel-hardening@...ts.openwall.com,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4] mm: SLAB freelist randomization
On Tue, Apr 26, 2016 at 04:17:43PM -0700, Andrew Morton wrote:
> On Tue, 26 Apr 2016 09:21:10 -0700 Thomas Garnier <thgarnie@...gle.com> wrote:
>
> > Provides an optional config (CONFIG_FREELIST_RANDOM) to randomize the
> > SLAB freelist. The list is randomized during initialization of a new set
> > of pages. The order on different freelist sizes is pre-computed at boot
> > for performance. Each kmem_cache has its own randomized freelist. Before
> > pre-computed lists are available freelists are generated
> > dynamically. This security feature reduces the predictability of the
> > kernel SLAB allocator against heap overflows rendering attacks much less
> > stable.
> >
> > For example this attack against SLUB (also applicable against SLAB)
> > would be affected:
> > https://jon.oberheide.org/blog/2010/09/10/linux-kernel-can-slub-overflow/
> >
> > Also, since v4.6 the freelist was moved at the end of the SLAB. It means
> > a controllable heap is opened to new attacks not yet publicly discussed.
> > A kernel heap overflow can be transformed to multiple use-after-free.
> > This feature makes this type of attack harder too.
> >
> > To generate entropy, we use get_random_bytes_arch because 0 bits of
> > entropy is available in the boot stage. In the worse case this function
> > will fallback to the get_random_bytes sub API. We also generate a shift
> > random number to shift pre-computed freelist for each new set of pages.
> >
> > The config option name is not specific to the SLAB as this approach will
> > be extended to other allocators like SLUB.
> >
> > Performance results highlighted no major changes:
> >
> > Hackbench (running 90 10 times):
> >
> > Before average: 0.0698
> > After average: 0.0663 (-5.01%)
> >
> > slab_test 1 run on boot. Difference only seen on the 2048 size test
> > being the worse case scenario covered by freelist randomization. New
> > slab pages are constantly being created on the 10000 allocations.
> > Variance should be mainly due to getting new pages every few
> > allocations.
> >
> > ...
> >
> > --- a/include/linux/slab_def.h
> > +++ b/include/linux/slab_def.h
> > @@ -80,6 +80,10 @@ struct kmem_cache {
> > struct kasan_cache kasan_info;
> > #endif
> >
> > +#ifdef CONFIG_FREELIST_RANDOM
> > + void *random_seq;
> > +#endif
> > +
> > struct kmem_cache_node *node[MAX_NUMNODES];
> > };
> >
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 0c66640..73453d0 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1742,6 +1742,15 @@ config SLOB
> >
> > endchoice
> >
> > +config FREELIST_RANDOM
> > + default n
> > + depends on SLAB
> > + bool "SLAB freelist randomization"
> > + help
> > + Randomizes the freelist order used on creating new SLABs. This
> > + security feature reduces the predictability of the kernel slab
> > + allocator against heap overflows.
>
> Against the v2 patch I didst observe:
>
> : CONFIG_FREELIST_RANDOM bugs me a bit - "freelist" is so vague.
> : CONFIG_SLAB_FREELIST_RANDOM would be better. I mean, what Kconfig
> : identifier could be used for implementing randomisation in
> : slub/slob/etc once CONFIG_FREELIST_RANDOM is used up?
>
> but this pearl appeared to pass unnoticed.
>
> > config SLUB_CPU_PARTIAL
> > default y
> > depends on SLUB && SMP
> > diff --git a/mm/slab.c b/mm/slab.c
> > index b82ee6b..0ed728a 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -1230,6 +1230,61 @@ static void __init set_up_node(struct kmem_cache *cachep, int index)
> > }
> > }
> >
> > +#ifdef CONFIG_FREELIST_RANDOM
> > +static void freelist_randomize(struct rnd_state *state, freelist_idx_t *list,
> > + size_t count)
> > +{
> > + size_t i;
> > + unsigned int rand;
> > +
> > + for (i = 0; i < count; i++)
> > + list[i] = i;
> > +
> > + /* Fisher-Yates shuffle */
> > + for (i = count - 1; i > 0; i--) {
> > + rand = prandom_u32_state(state);
> > + rand %= (i + 1);
> > + swap(list[i], list[rand]);
> > + }
> > +}
> > +
> > +/* Create a random sequence per cache */
> > +static int cache_random_seq_create(struct kmem_cache *cachep)
> > +{
> > + unsigned int seed, count = cachep->num;
> > + struct rnd_state state;
> > +
> > + if (count < 2)
> > + return 0;
> > +
> > + /* If it fails, we will just use the global lists */
> > + cachep->random_seq = kcalloc(count, sizeof(freelist_idx_t), GFP_KERNEL);
> > + if (!cachep->random_seq)
> > + return -ENOMEM;
>
> OK, no BUG. If this happens, kmem_cache_init_late() will go BUG
> instead ;)
>
> Questions for slab maintainers:
>
> What's going on with the gfp_flags in there? kmem_cache_init_late()
> passes GFP_NOWAIT into enable_cpucache().
>
> a) why the heck does it do that? It's __init code!
Until some boot-up point, we should not enable interrupt.
In slab subsystem, If we use __GFP_DIRECT_RECLAIM, it will cause to
enable interrupt when allocating new slab page. GFP_NOWAIT is to
prevent that situation.
Anyway, I audit the code and kmem_cache_init_late() could use
__GFP_DIRECT_RECLAIM because it is called after interrupt is enabled
which means that that's safe time to manipulate interrupt. (See
kmem_cache_init_late() in start_kernel()).
>
> b) if there's a legit reason then your new cache_random_seq_create()
> should be getting its gfp_t from its caller, rather than blindly
> assuming GFP_KERNEL.
In any case, ignoring provided gfp argument isn't good practice.
> c) kmem_cache_init_late() goes BUG on ENOMEM. Generally that's OK in
> __init code: we assume infinite memory during bootup. But it's really
> quite weird to use GFP_NOWAIT and then to go BUG if GFP_NOWAIT had its
> predictable outcome (ie: failure).
I don't think BUG() here is weird code. It just means that if we can't
initialize slab subsystem properly, machine cannot run properly so
BUG().
> Finally, all callers of enable_cpucache() (and hence of
> cache_random_seq_create()) are __init, so we're unnecessarily bloating
> up vmlinux. Could someone please take a look at this as a separate
> thing?
That's not true. It is called whenever new kmem_cache is created.
I don't know concrete reason why setup_cpu_cache() is defined with
__init_refok tag but looks like it needs to be fixed.
I will look at it soon.
Thanks.
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