Re: [PATCH v2] nsproxy: Convert nsproxy.count to refcount_t

[Christian Brauner]

On Thu, Aug 17, 2023 at 09:13:32PM -0700, Kees Cook 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.
> 
> The variable nsproxy.count is used as pure reference counter. Convert it
> to refcount_t and fix up the operations.
> 
> **Important note for maintainers:
> 
> Some functions from refcount_t API defined in refcount.h have different
> memory ordering guarantees than their atomic counterparts. Please check
> Documentation/core-api/refcount-vs-atomic.rst for more information.
> 
> Normally the differences should not matter since refcount_t provides
> enough guarantees to satisfy the refcounting use cases, but in some
> rare cases it might matter. Please double check that you don't have
> some undocumented memory guarantees for this variable usage.
> 
> For the nsproxy.count it might make a difference in following places:
>  - put_nsproxy() and switch_task_namespaces(): decrement in
>    refcount_dec_and_test() only provides RELEASE ordering and ACQUIRE
>    ordering on success vs. fully ordered atomic counterpart
> 
> Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
> Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---

Reviewed-by: Christian Brauner <brauner@xxxxxxxxxx>