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?
