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SubjectRe: [PATCH] mm: Add SLUB free list pointer obfuscation
On Tue, Jun 20, 2017 at 11:05 AM, Laura Abbott <labbott@redhat.com> wrote:
> On 06/19/2017 08:01 PM, Kees Cook wrote:
>> This SLUB free list pointer obfuscation code is modified from Brad
>> Spengler/PaX Team's code in the last public patch of grsecurity/PaX based
>> on my understanding of the code. Changes or omissions from the original
>> code are mine and don't reflect the original grsecurity/PaX code.
>>
>> This adds a per-cache random value to SLUB caches that is XORed with
>> their freelist pointers. This adds nearly zero overhead and frustrates the
>> very common heap overflow exploitation method of overwriting freelist
>> pointers. A recent example of the attack is written up here:
>> http://cyseclabs.com/blog/cve-2016-6187-heap-off-by-one-exploit
>>
>> This is based on patches by Daniel Micay, and refactored to avoid lots
>> of #ifdef code.
>>
>> Suggested-by: Daniel Micay <danielmicay@gmail.com>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>> include/linux/slub_def.h | 4 ++++
>> init/Kconfig | 10 ++++++++++
>> mm/slub.c | 32 +++++++++++++++++++++++++++-----
>> 3 files changed, 41 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
>> index 07ef550c6627..0258d6d74e9c 100644
>> --- a/include/linux/slub_def.h
>> +++ b/include/linux/slub_def.h
>> @@ -93,6 +93,10 @@ struct kmem_cache {
>> #endif
>> #endif
>>
>> +#ifdef CONFIG_SLAB_HARDENED
>> + unsigned long random;
>> +#endif
>> +
>> #ifdef CONFIG_NUMA
>> /*
>> * Defragmentation by allocating from a remote node.
>> diff --git a/init/Kconfig b/init/Kconfig
>> index 1d3475fc9496..eb91082546bf 100644
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -1900,6 +1900,16 @@ config SLAB_FREELIST_RANDOM
>> security feature reduces the predictability of the kernel slab
>> allocator against heap overflows.
>>
>> +config SLAB_HARDENED
>> + bool "Harden slab cache infrastructure"
>> + default y
>> + depends on SLAB_FREELIST_RANDOM && SLUB> + help
>> + Many kernel heap attacks try to target slab cache metadata and
>> + other infrastructure. This options makes minor performance
>> + sacrifies to harden the kernel slab allocator against common
>> + exploit methods.
>> +
>
> Going to bikeshed on SLAB_HARDENED unless this is intended to be used for
> more things. Perhaps SLAB_FREELIST_HARDENED?

Daniel's tree has a bunch of changes attached to that config name, but
it's unclear to me how many would be accepted upstream. I would be
fine with SLAB_FREELIST_HARDENED.

> What's the reason for the dependency on SLAB_FREELIST_RANDOM?

Looking at it again, I suspect the idea was to collect other configs
under SLAB_HARDENED. It should likely be either be a select or just
dropped.

-Kees

--
Kees Cook
Pixel Security

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