Re: [PATCH v5 2/6] mm/slab: Plumb kmem_buckets into __do_kmalloc_node()

[Vlastimil Babka <vbabka@suse.cz>]

On 6/19/24 9:33 PM, Kees Cook wrote:
> Introduce CONFIG_SLAB_BUCKETS which provides the infrastructure to
> support separated kmalloc buckets (in the following kmem_buckets_create()
> patches and future codetag-based separation). Since this will provide
> a mitigation for a very common case of exploits, enable it by default.

No longer "enable it by default".

> 
> To be able to choose which buckets to allocate from, make the buckets
> available to the internal kmalloc interfaces by adding them as the
> first argument, rather than depending on the buckets being chosen from

second argument now

> the fixed set of global buckets. Where the bucket is not available,
> pass NULL, which means "use the default system kmalloc bucket set"
> (the prior existing behavior), as implemented in kmalloc_slab().
> 
> To avoid adding the extra argument when !CONFIG_SLAB_BUCKETS, only the
> top-level macros and static inlines use the buckets argument (where
> they are stripped out and compiled out respectively). The actual extern
> functions can then been built without the argument, and the internals
> fall back to the global kmalloc buckets unconditionally.

Also describes the previous implementation and not the new one?

> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -273,6 +273,22 @@ config SLAB_FREELIST_HARDENED
>  	  sacrifices to harden the kernel slab allocator against common
>  	  freelist exploit methods.
>  
> +config SLAB_BUCKETS
> +	bool "Support allocation from separate kmalloc buckets"
> +	depends on !SLUB_TINY
> +	help
> +	  Kernel heap attacks frequently depend on being able to create
> +	  specifically-sized allocations with user-controlled contents
> +	  that will be allocated into the same kmalloc bucket as a
> +	  target object. To avoid sharing these allocation buckets,
> +	  provide an explicitly separated set of buckets to be used for
> +	  user-controlled allocations. This may very slightly increase
> +	  memory fragmentation, though in practice it's only a handful
> +	  of extra pages since the bulk of user-controlled allocations
> +	  are relatively long-lived.
> +
> +	  If unsure, say Y.

I was wondering why I don't see the buckets in slabinfo and turns out it was
SLAB_MERGE_DEFAULT. It would probably make sense for SLAB_MERGE_DEFAULT to
depends on !SLAB_BUCKETS now as the merging defeats the purpose, wdyt?