Re: [kernel-hardening] [PATCH 23/23] mm: Allow slab_nomerge to be set at build time

[Kees Cook <keescook@chromium.org>]

On Mon, Jun 19, 2017 at 9:29 PM, Eric Biggers <ebiggers3@gmail.com> wrote:
> 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@chromium.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.

I've split it out and updated the docs, thanks!

-Kees

-- 
Kees Cook
Pixel Security