Re: [PATCH v2 0/9] slab: Introduce dedicated bucket allocator

[GONG, Ruiqi]

On 2024/03/08 4:31, Kees Cook wrote:
> On Wed, Mar 06, 2024 at 09:47:36AM +0800, GONG, Ruiqi wrote:
>>
>>
>> On 2024/03/05 18:10, Kees Cook wrote:
>>> Hi,
>>>
>>> Repeating the commit logs for patch 4 here:
>>>
>>>     Dedicated caches are available For fixed size allocations via
>>>     kmem_cache_alloc(), but for dynamically sized allocations there is only
>>>     the global kmalloc API's set of buckets available. This means it isn't
>>>     possible to separate specific sets of dynamically sized allocations into
>>>     a separate collection of caches.
>>>
>>>     This leads to a use-after-free exploitation weakness in the Linux
>>>     kernel since many heap memory spraying/grooming attacks depend on using
>>>     userspace-controllable dynamically sized allocations to collide with
>>>     fixed size allocations that end up in same cache.
>>>
>>>     While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
>>>     against these kinds of "type confusion" attacks, including for fixed
>>>     same-size heap objects, we can create a complementary deterministic
>>>     defense for dynamically sized allocations.
>>>
>>>     In order to isolate user-controllable sized allocations from system
>>>     allocations, introduce kmem_buckets_create(), which behaves like
>>>     kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
>>>     which behaves like kmem_cache_alloc().)
>>
>> So can I say the vision here would be to make all the kernel interfaces
>> that handles user space input to use separated caches? Which looks like
>> creating a "grey zone" in the middle of kernel space (trusted) and user
>> space (untrusted) memory. I've also thought that maybe hardening on the
>> "border" could be more efficient and targeted than a mitigation that
>> affects globally, e.g. CONFIG_RANDOM_KMALLOC_CACHES.
> 
> I think it ends up having a similar effect, yes. The more copies that
> move to memdup_user(), the more coverage is created. The main point is to
> just not share caches between different kinds of allocations. The most
> abused version of this is the userspace size-controllable allocations,
> which this targets. 

I agree. Currently if we want to fulfill a more strict separation
between user-space manageable memory and other memory in kernel space,
technically speaking for fixed size allocations we could transform them
into using dedicated caches (i.e. kmem_cache_create()), but for dynamic
size allocations I don't think of any solution. With the APIs provided
by this patch set, we've got something that works.


> ... The existing caches (which could still be used for
> type confusion attacks when the sizes are sufficiently similar) have a
> good chance of being mitigated by CONFIG_RANDOM_KMALLOC_CACHES already,
> so this proposed change is just complementary, IMO.

Maybe in the future we could require that all user-kernel interfaces
that make use of SLAB caches should use either kmem_cache_create() or
kmem_buckets_create()? ;)

> 
> -Kees
>