From: Michal Hocko <mhocko@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: Christoph Lameter <cl@linux.com>,
Jonathan Corbet <corbet@lwn.net>,
Daniel Micay <danielmicay@gmail.com>,
David Windsor <dave@nullcore.net>,
Eric Biggers <ebiggers3@gmail.com>,
Pekka Enberg <penberg@kernel.org>,
David Rientjes <rientjes@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Andrew Morton <akpm@linux-foundation.org>,
"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>,
Mauro Carvalho Chehab <mchehab@kernel.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Arnd Bergmann <arnd@arndb.de>, Andy Lutomirski <luto@kernel.org>,
Nicolas Pitre <nicolas.pitre@linaro.org>,
Tejun Heo <tj@kernel.org>, Daniel Mack <daniel@zonque.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Helge Deller <deller@gmx.de>, Rik van Riel <riel@redhat.com>,
linux-doc@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm: Allow slab_nomerge to be set at build time
Date: Fri, 23 Jun 2017 16:06:51 +0200 [thread overview]
Message-ID: <20170623140651.GD5314@dhcp22.suse.cz> (raw)
In-Reply-To: <20170620230911.GA25238@beast>
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
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
--
Michal Hocko
SUSE Labs
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-06-23 14:06 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-20 23:09 [PATCH v2] mm: Allow slab_nomerge to be set at build time Kees Cook
2017-06-20 23:16 ` Randy Dunlap
2017-06-20 23:29 ` Kees Cook
2017-06-21 0:09 ` Randy Dunlap
2017-06-23 14:06 ` Michal Hocko [this message]
2017-06-23 19:20 ` Kees Cook
2017-06-26 9:00 ` Michal Hocko
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170623140651.GD5314@dhcp22.suse.cz \
--to=mhocko@kernel.org \
--cc=akpm@linux-foundation.org \
--cc=arnd@arndb.de \
--cc=bigeasy@linutronix.de \
--cc=cl@linux.com \
--cc=corbet@lwn.net \
--cc=daniel@zonque.org \
--cc=danielmicay@gmail.com \
--cc=dave@nullcore.net \
--cc=deller@gmx.de \
--cc=ebiggers3@gmail.com \
--cc=iamjoonsoo.kim@lge.com \
--cc=keescook@chromium.org \
--cc=linux-doc@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=luto@kernel.org \
--cc=mchehab@kernel.org \
--cc=mingo@kernel.org \
--cc=nicolas.pitre@linaro.org \
--cc=paulmck@linux.vnet.ibm.com \
--cc=penberg@kernel.org \
--cc=rafael.j.wysocki@intel.com \
--cc=riel@redhat.com \
--cc=rientjes@google.com \
--cc=sergey.senozhatsky@gmail.com \
--cc=tglx@linutronix.de \
--cc=tj@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).