Re: [PATCH v2 4/9] slab: Introduce kmem_buckets_create()

[Kent Overstreet]

On Tue, Mar 05, 2024 at 02:10:20AM -0800, Kees Cook wrote:
> 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().)
> 
> Allows for confining allocations to a dedicated set of sized caches
> (which have the same layout as the kmalloc caches).
> 
> This can also be used in the future once codetag allocation annotations
> exist to implement per-caller allocation cache isolation[1] even for
> dynamic allocations.
> 
> Link: https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook [1]
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> Cc: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> ---
>  include/linux/slab.h |  5 +++
>  mm/slab_common.c     | 72 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 77 insertions(+)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index f26ac9a6ef9f..058d0e3cd181 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -493,6 +493,11 @@ void *kmem_cache_alloc_lru(struct kmem_cache *s, struct list_lru *lru,
>  			   gfp_t gfpflags) __assume_slab_alignment __malloc;
>  void kmem_cache_free(struct kmem_cache *s, void *objp);
>  
> +kmem_buckets *kmem_buckets_create(const char *name, unsigned int align,
> +				  slab_flags_t flags,
> +				  unsigned int useroffset, unsigned int usersize,
> +				  void (*ctor)(void *));

I'd prefer an API that initialized an object over one that allocates it
- that is, prefer

kmem_buckets_init(kmem_buckets *bucekts, ...)

by forcing it to be separately allocated, you're adding a pointer deref
to every access.

That would also allow for kmem_buckets to be lazily initialized, which
would play nicely declaring the kmem_buckets in the alloc_hooks() macro.

I'm curious what all the arguments to kmem_buckets_create() are needed
for, if this is supposed to be a replacement for kmalloc() users.