Re: [PATCH RFC] slab: support for compiler-assisted type-based slab cache partitioning

[Marco Elver <elver@google.com>]

On Tue, 26 Aug 2025 at 06:59, GONG Ruiqi <gongruiqi1@huawei.com> wrote:
> On 8/25/2025 11:44 PM, Marco Elver wrote:
> > ...
> >
> > Introduce a new mode, TYPED_KMALLOC_CACHES, which leverages Clang's
> > "allocation tokens" via __builtin_alloc_token_infer [1].
> >
> > This mechanism allows the compiler to pass a token ID derived from the
> > allocation's type to the allocator. The compiler performs best-effort
> > type inference, and recognizes idioms such as kmalloc(sizeof(T), ...).
> > Unlike RANDOM_KMALLOC_CACHES, this mode deterministically assigns a slab
> > cache to an allocation of type T, regardless of allocation site.
> >
> > Clang's default token ID calculation is described as [1]:
> >
> >    TypeHashPointerSplit: This mode assigns a token ID based on the hash
> >    of the allocated type's name, where the top half ID-space is reserved
> >    for types that contain pointers and the bottom half for types that do
> >    not contain pointers.
>
> Is a type's token id always the same across different builds? Or somehow
> predictable? If so, the attacker could probably find out all types that
> end up with the same id, and use some of them to exploit the buggy one.

Yes, it's meant to be deterministic and predictable. I guess this is
the same question regarding randomness, for which it's unclear if it
strengthens or weakens the mitigation. As I wrote elsewhere:

> Irrespective of the top/bottom split, one of the key properties to
> retain is that allocations of type T are predictably assigned a slab
> cache. This means that even if a pointer-containing object of type T
> is vulnerable, yet the pointer within T is useless for exploitation,
> the difficulty of getting to a sensitive object S is still increased
> by the fact that S is unlikely to be co-located. If we were to
> introduce more randomness, we increase the probability that S will be
> co-located with T, which is counter-intuitive to me.

I think we can reason either way, and I grant you this is rather ambiguous.

But the definitive point that was made to me from various security
researchers that inspired this technique is that the most useful thing
we can do is separate pointer-containing objects from
non-pointer-containing objects (in absence of slab per type, which is
likely too costly in the common case).