Re: [PATCH RFC v2] Randomized slab caches for kmalloc()

[Pedro Falcato]

On Wed, May 10, 2023 at 7:44 PM Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote:
>
> On Mon, May 8, 2023 at 12:53 AM GONG, Ruiqi <gongruiqi1@xxxxxxxxxx> 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 address 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.
> >
> > The overhead of performance has been tested on a 40-core x86 server by
> > comparing the results of `perf bench all` between the kernels with and
> > without this patch based on the latest linux-next kernel, which shows
> > minor difference. A subset of benchmarks are listed below:
> >
>
> Please Cc maintainers/reviewers of corresponding subsystem in MAINTAINERS file.
>
> I dont think adding a hardening feature by sacrificing one digit
> percent performance
> (and additional complexity) is worth. Heap spraying can only occur
> when the kernel contains
> security vulnerabilities, and if there is no known ways of performing
> such an attack,
> then we would simply be paying a consistent cost.
>

And does the kernel not contain security vulnerabilities? :v
This feature is opt-in and locked behind a CONFIG_ and the kernel most
certainly has security vulnerabilities.

So... I don't see why adding the hardening feature would be a bad
idea, barring it being a poor hardening feature, the patch being poor
or the complexity being overwhelming.

-- 
Pedro