Re: [PATCH RFC] Randomized slab caches for kmalloc()

[Hyeonggon Yoo]

On 3/15/2023 6:54 PM, GONG, Ruiqi wrote:
> When exploiting memory vulnerabilities, "heap spraying" is a common
> technique targeting those related to dynamic memory allocation (i.e. the
> "heap"), and it plays an important role in a successful exploitation.
> Basically, it is to overwrite the memory area of vulnerable object by
> triggering allocation in other subsystems or modules and therefore
> getting a reference to the targeted memory location. It's usable on
> various types of vulnerablity including use after free (UAF), heap out-
> of-bound write and etc.
>
> There are (at least) two reasons why the heap can be sprayed: 1) generic
> slab caches are shared among different subsystems and modules, and
> 2) dedicated slab caches could be merged with the generic ones.
> Currently these two factors cannot be prevented at a low cost: the first
> one is a widely used memory allocation mechanism, and shutting down slab
> merging completely via `slub_nomerge` would be overkill.
>
> To efficiently prevent heap spraying, we propose the following approach:
> to create multiple copies of generic slab caches that will never be
> merged, and random one of them will be used at allocation. The random
> selection is based on the location of code that calls `kmalloc()`, which
> means it is static at runtime (rather than dynamically determined at
> each time of allocation, which could be bypassed by repeatedly spraying
> in brute force). In this way, the vulnerable object and memory allocated
> in other subsystems and modules will (most probably) be on different
> slab caches, which prevents the object from being sprayed.
>
> Signed-off-by: GONG, Ruiqi <gongruiqi1@xxxxxxxxxx>
> ---

I'm not yet sure if this feature is appropriate for mainline kernel.

I have few questions:

1) What is cost of this configuration, in terms of memory overhead, or 
execution time?


2) The actual cache depends on caller which is static at build time, not 
runtime.

    What about using (caller ^ (some subsystem-wide random sequence)),

    which is static at runtime?