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

Randy Dunlap
2017-06-21T00:09:20Z

On 06/20/2017 04:29 PM, Kees Cook wrote:
> On Tue, Jun 20, 2017 at 4:16 PM, Randy Dunlap <rdunlap@infradead.org> wrote:
>> On 06/20/2017 04:09 PM, 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).
>>>
>>> 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/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.
>>
>> command line or this option can be disabled in the kernel config.
>
> Isn't that implicit in that it is Kconfig help text? Happy to add it,
> but seems redundant to me.
>

Just trying for completeness instead of being implicit.

>
>>
>>> +
>>> config SLAB_FREELIST_RANDOM
>>> default n
>>> depends on SLAB || SLUB
>>
>> --
>> ~Randy
>
>
>

--
~Randy