Re: [PATCH v4 14/36] lib: add allocation tagging support for memory allocation profiling

Kees Cook
2024-02-21T23:05:34Z

On Wed, Feb 21, 2024 at 11:40:27AM -0800, Suren Baghdasaryan wrote:
> [...]
> +struct alloc_tag {
> + struct codetag ct;
> + struct alloc_tag_counters __percpu *counters;
> +} __aligned(8);
> [...]
> +#define DEFINE_ALLOC_TAG(_alloc_tag) \
> + static DEFINE_PER_CPU(struct alloc_tag_counters, _alloc_tag_cntr); \
> + static struct alloc_tag _alloc_tag __used __aligned(8) \
> + __section("alloc_tags") = { \
> + .ct = CODE_TAG_INIT, \
> + .counters = &_alloc_tag_cntr };
> [...]
> +static inline struct alloc_tag *alloc_tag_save(struct alloc_tag *tag)
> +{
> + swap(current->alloc_tag, tag);
> + return tag;
> +}

Future security hardening improvement idea based on this infrastructure:
it should be possible to implement per-allocation-site kmem caches. For
example, we could create:

struct alloc_details {
u32 flags;
union {
u32 size; /* not valid after __init completes */
struct kmem_cache *cache;
};
};

- add struct alloc_details to struct alloc_tag
- move the tags section into .ro_after_init
- extend alloc_hooks() to populate flags and size:
.flags = __builtin_constant_p(size) ? KMALLOC_ALLOCATE_FIXED
: KMALLOC_ALLOCATE_BUCKETS;
.size = __builtin_constant_p(size) ? size : SIZE_MAX;
- during kernel start or module init, walk the alloc_tag list
and create either a fixed-size kmem_cache or to allocate a
full set of kmalloc-buckets, and update the "cache" member.
- adjust kmalloc core routines to use current->alloc_tag->cache instead
of using the global buckets.

This would get us fully separated allocations, producing better than
type-based levels of granularity, exceeding what we have currently with
CONFIG_RANDOM_KMALLOC_CACHES.

Does this look possible, or am I misunderstanding something in the
infrastructure being created here?

--
Kees Cook