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Date: Wed, 21 Feb 2024 15:05:32 -0800
From: Kees Cook <keescook@chromium.org>
To: Suren Baghdasaryan <surenb@google.com>
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Subject: Re: [PATCH v4 14/36] lib: add allocation tagging support for memory
 allocation profiling
Message-ID: <202402211449.401382D2AF@keescook>
References: <20240221194052.927623-1-surenb@google.com>
 <20240221194052.927623-15-surenb@google.com>
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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