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

Michal Hocko
2017-06-23T14:07:00Z

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