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Date: Mon, 19 Jun 2017 21:29:36 -0700
From: Eric Biggers <ebiggers3@...il.com>
To: Kees Cook <keescook@...omium.org>
Cc: kernel-hardening@...ts.openwall.com, David Windsor <dave@...lcore.net>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 23/23] mm: Allow slab_nomerge to be
set at build time
On Mon, Jun 19, 2017 at 04:36:37PM -0700, 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, 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).
>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> mm/slab_common.c | 5 ++---
> security/Kconfig | 13 +++++++++++++
> 2 files changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 6c14d765379f..17a4c4b33283 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;
> }
>
> diff --git a/security/Kconfig b/security/Kconfig
> index 0c181cebdb8a..e40bd2a260f8 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -166,6 +166,19 @@ config HARDENED_USERCOPY_SPLIT_KMALLOC
> confined to a separate cache, attackers must find other ways
> to prepare heap attacks that will be near their desired target.
>
> +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 small risk of kernel heap overflows being able
> + to overwrite objects from merged caches, which reduces the
> + difficulty of such heap attacks. 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.
> +
It's good to at least have this option, but again it's logically separate and
shouldn't just be hidden in patch 23/23. And again, is it really just about
heap overflows?
Please also fix the documentation for slab_nomerge in
Documentation/admin-guide/kernel-parameters.txt.
- Eric
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