linux-hardening - Re: [PATCH v3 1/1] Randomized slab caches for kmalloc()

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Date:   Thu, 22 Jun 2023 15:56:04 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     "GONG, Ruiqi" <gongruiqi@...weicloud.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Christoph Lameter <cl@...ux.com>, Tejun Heo <tj@...nel.org>,
        Dennis Zhou <dennis@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Lobakin <aleksander.lobakin@...el.com>,
        Pedro Falcato <pedro.falcato@...il.com>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Wang Weiyang <wangweiyang2@...wei.com>,
        Xiu Jianfeng <xiujianfeng@...wei.com>, linux-mm@...ck.org,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        gongruiqi1@...wei.com
Subject: Re: [PATCH v3 1/1] Randomized slab caches for kmalloc()

On 6/16/23 13:18, GONG, Ruiqi wrote:
> index a3c95338cd3a..6150e9a946a7 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -337,6 +337,55 @@ config SLUB_CPU_PARTIAL
>  	  which requires the taking of locks that may cause latency spikes.
>  	  Typically one would choose no for a realtime system.
>  
> +config RANDOM_KMALLOC_CACHES
> +	default n
> +	depends on SLUB
> +	bool "Random slab caches for normal kmalloc"
> +	help
> +	  A hardening feature that creates multiple copies of slab caches for
> +	  normal kmalloc allocation and makes kmalloc randomly pick one based
> +	  on code address, which makes the attackers unable to spray vulnerable
> +	  memory objects on the heap for exploiting memory vulnerabilities.
> +
> +choice
> +	prompt "Number of random slab caches copies"
> +	depends on RANDOM_KMALLOC_CACHES
> +	default RANDOM_KMALLOC_CACHES_16
> +	help
> +	  The number of copies of random slab caches. Bigger value makes the
> +	  potentially vulnerable memory object less likely to collide with
> +	  objects allocated from other subsystems or modules.

When I read this, without further knowledge, why would I select anything
else than the largest value? It should mention memory overhead maybe?

Also would anyone really select only "2" and thus limit the collision
probability to 50% and not less? "4" also seems quite low for the given
purpose? Could we just pick and hardcode 8 or 16 and avoid the selection, at
least until there's some more experience with the whole approach?