Re: [PATCH v2] mm: Allow slab_nomerge to be set at build time

[Michal Hocko <mhocko@kernel.org>]

On Tue 20-06-17 16:09:11, Kees Cook wrote:
> Some hardened environments want to build kernels with slab_nomerge
> already set (so that they do not depend on remembering to set the kernel
> command line option). This is desired to reduce the risk of kernel heap
> overflows being able to overwrite objects from merged caches and changes
> the requirements for cache layout control, increasing the difficulty of
> these attacks. By keeping caches unmerged, these kinds of exploits can
> usually only damage objects in the same cache (though the risk to metadata
> exploitation is unchanged).

Do we really want to have a dedicated config for each hardening specific
kernel command line? I believe we have quite a lot of config options
already. Can we rather have a CONFIG_HARDENED_CMD_OPIONS and cover all
those defauls there instead?

> Cc: Daniel Micay <danielmicay@gmail.com>
> Cc: David Windsor <dave@nullcore.net>
> Cc: Eric Biggers <ebiggers3@gmail.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v2: split out of slab whitelisting series
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 10 ++++++++--
>  init/Kconfig                                    | 14 ++++++++++++++
>  mm/slab_common.c                                |  5 ++---
>  3 files changed, 24 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 7737ab5d04b2..94d8b8195cb8 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3715,8 +3715,14 @@
>  	slab_nomerge	[MM]
>  			Disable merging of slabs with similar size. May be
>  			necessary if there is some reason to distinguish
> -			allocs to different slabs. Debug options disable
> -			merging on their own.
> +			allocs to different slabs, especially in hardened
> +			environments where the risk of heap overflows and
> +			layout control by attackers can usually be
> +			frustrated by disabling merging. This will reduce
> +			most of the exposure of a heap attack to a single
> +			cache (risks via metadata attacks are mostly
> +			unchanged). Debug options disable merging on their
> +			own.
>  			For more information see Documentation/vm/slub.txt.
>  
>  	slab_max_order=	[MM, SLAB]
> diff --git a/init/Kconfig b/init/Kconfig
> index 1d3475fc9496..ce813acf2f4f 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1891,6 +1891,20 @@ config SLOB
>  
>  endchoice
>  
> +config SLAB_MERGE_DEFAULT
> +	bool "Allow slab caches to be merged"
> +	default y
> +	help
> +	  For reduced kernel memory fragmentation, slab caches can be
> +	  merged when they share the same size and other characteristics.
> +	  This carries a risk of kernel heap overflows being able to
> +	  overwrite objects from merged caches (and more easily control
> +	  cache layout), which makes such heap attacks easier to exploit
> +	  by attackers. By keeping caches unmerged, these kinds of exploits
> +	  can usually only damage objects in the same cache. To disable
> +	  merging at runtime, "slab_nomerge" can be passed on the kernel
> +	  command line.
> +
>  config SLAB_FREELIST_RANDOM
>  	default n
>  	depends on SLAB || SLUB
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 01a0fe2eb332..904a83be82de 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -47,13 +47,12 @@ static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
>  
>  /*
>   * Merge control. If this is set then no merging of slab caches will occur.
> - * (Could be removed. This was introduced to pacify the merge skeptics.)
>   */
> -static int slab_nomerge;
> +static bool slab_nomerge = !IS_ENABLED(CONFIG_SLAB_MERGE_DEFAULT);
>  
>  static int __init setup_slab_nomerge(char *str)
>  {
> -	slab_nomerge = 1;
> +	slab_nomerge = true;
>  	return 1;
>  }
>  
> -- 
> 2.7.4
> 
> 
> -- 
> Kees Cook
> Pixel Security
> 
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-- 
Michal Hocko
SUSE Labs