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

Andrew Morton
2023-08-18T17:56:39Z

On Thu, 17 Aug 2023 21:17:41 -0700 Kees Cook <keescook@chromium.org> wrote:

> From: Elena Reshetova <elena.reshetova@intel.com>
>
> 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? Or something like that. dangnabbit, that refcount_t.

x86_64 defconfig:

before:
text data bss dec hex filename
3869 552 8 4429 114d kernel/cred.o
6140 724 16 6880 1ae0 net/sunrpc/auth.o

after:
text data bss dec hex filename
4573 552 8 5133 140d kernel/cred.o
6236 724 16 6976 1b40 net/sunrpc/auth.o

Please explain, in a non handwavy and non cargoculty fashion why this
speed and space cost is justified.