[kernel-hardening] Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support

[Laura Abbott <labbott@redhat.com>]

On 07/25/2016 01:45 PM, Kees Cook wrote:
> On Mon, Jul 25, 2016 at 12:16 PM, Laura Abbott <labbott@redhat.com> wrote:
>> On 07/20/2016 01:27 PM, Kees Cook wrote:
>>>
>>> Under CONFIG_HARDENED_USERCOPY, this adds object size checking to the
>>> SLUB allocator to catch any copies that may span objects. Includes a
>>> redzone handling fix discovered by Michael Ellerman.
>>>
>>> Based on code from PaX and grsecurity.
>>>
>>> Signed-off-by: Kees Cook <keescook@chromium.org>
>>> Tested-by: Michael Ellerman <mpe@ellerman.id.au>
>>> ---
>>>  init/Kconfig |  1 +
>>>  mm/slub.c    | 36 ++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 37 insertions(+)
>>>
>>> diff --git a/init/Kconfig b/init/Kconfig
>>> index 798c2020ee7c..1c4711819dfd 100644
>>> --- a/init/Kconfig
>>> +++ b/init/Kconfig
>>> @@ -1765,6 +1765,7 @@ config SLAB
>>>
>>>  config SLUB
>>>         bool "SLUB (Unqueued Allocator)"
>>> +       select HAVE_HARDENED_USERCOPY_ALLOCATOR
>>>         help
>>>            SLUB is a slab allocator that minimizes cache line usage
>>>            instead of managing queues of cached objects (SLAB approach).
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index 825ff4505336..7dee3d9a5843 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -3614,6 +3614,42 @@ void *__kmalloc_node(size_t size, gfp_t flags, int
>>> node)
>>>  EXPORT_SYMBOL(__kmalloc_node);
>>>  #endif
>>>
>>> +#ifdef CONFIG_HARDENED_USERCOPY
>>> +/*
>>> + * Rejects objects that are incorrectly sized.
>>> + *
>>> + * Returns NULL if check passes, otherwise const char * to name of cache
>>> + * to indicate an error.
>>> + */
>>> +const char *__check_heap_object(const void *ptr, unsigned long n,
>>> +                               struct page *page)
>>> +{
>>> +       struct kmem_cache *s;
>>> +       unsigned long offset;
>>> +       size_t object_size;
>>> +
>>> +       /* Find object and usable object size. */
>>> +       s = page->slab_cache;
>>> +       object_size = slab_ksize(s);
>>> +
>>> +       /* Find offset within object. */
>>> +       offset = (ptr - page_address(page)) % s->size;
>>> +
>>> +       /* Adjust for redzone and reject if within the redzone. */
>>> +       if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
>>> +               if (offset < s->red_left_pad)
>>> +                       return s->name;
>>> +               offset -= s->red_left_pad;
>>> +       }
>>> +
>>> +       /* Allow address range falling entirely within object size. */
>>> +       if (offset <= object_size && n <= object_size - offset)
>>> +               return NULL;
>>> +
>>> +       return s->name;
>>> +}
>>> +#endif /* CONFIG_HARDENED_USERCOPY */
>>> +
>>
>>
>> I compared this against what check_valid_pointer does for SLUB_DEBUG
>> checking. I was hoping we could utilize that function to avoid
>> duplication but a) __check_heap_object needs to allow accesses anywhere
>> in the object, not just the beginning b) accessing page->objects
>> is racy without the addition of locking in SLUB_DEBUG.
>>
>> Still, the ptr < page_address(page) check from __check_heap_object would
>> be good to add to avoid generating garbage large offsets and trying to
>> infer C math.
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 7dee3d9..5370e4f 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -3632,6 +3632,9 @@ const char *__check_heap_object(const void *ptr,
>> unsigned long n,
>>         s = page->slab_cache;
>>         object_size = slab_ksize(s);
>>  +       if (ptr < page_address(page))
>> +               return s->name;
>> +
>>         /* Find offset within object. */
>>         offset = (ptr - page_address(page)) % s->size;
>>
>> With that, you can add
>>
>> Reviwed-by: Laura Abbott <labbott@redhat.com>
>
> Cool, I'll add that.
>
> Should I add your reviewed-by for this patch only or for the whole series?
>
> Thanks!
>
> -Kees
>

Just this patch for now, I'm working through a couple of others

>>
>>>  static size_t __ksize(const void *object)
>>>  {
>>>         struct page *page;
>>>
>>
>> Thanks,
>> Laura
>
>
>