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

[GONG, Ruiqi]

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.