Re: [PATCH v2] creds: Convert cred.usage to refcount_t

[Jann Horn]

On Fri, Aug 18, 2023 at 7:56 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 17 Aug 2023 21:17:41 -0700 Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> > From: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> >
> > atomic_t variables are currently used to implement reference counters
> > with the following properties:
> >  - counter is initialized to 1 using atomic_set()
> >  - a resource is freed upon counter reaching zero
> >  - once counter reaches zero, its further
> >    increments aren't allowed
> >  - counter schema uses basic atomic operations
> >    (set, inc, inc_not_zero, dec_and_test, etc.)
> >
> > Such atomic variables should be converted to a newly provided
> > refcount_t type and API that prevents accidental counter overflows and
> > underflows. This is important since overflows and underflows can lead
> > to use-after-free situation and be exploitable.
>
> ie, if we have bugs which we have no reason to believe presently exist,
> let's bloat and slow down the kernel just in case we add some in the
> future?

Yeah. Or in case we currently have some that we missed.

Though really we don't *just* need refcount_t to catch bugs; on a
system with enough RAM you can also overflow many 32-bit refcounts by
simply creating 2^32 actual references to an object. Depending on the
structure of objects that hold such refcounts, that can start
happening at around 2^32 * 8 bytes = 32 GiB memory usage, and it
becomes increasingly practical to do this with more objects if you
have significantly more RAM. I suppose you could avoid such issues by
putting a hard limit of 32 GiB on the amount of slab memory and
requiring that kernel object references are stored as pointers in slab
memory, or by making all the refcounts 64-bit.