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SubjectRe: [PATCH v2] nsproxy: Convert nsproxy.count to refcount_t
On Thu, Aug 17, 2023 at 09:13:32PM -0700, Kees Cook 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.
>
> 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@chromium.org>
> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> Reviewed-by: David Windsor <dwindsor@gmail.com>
> Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---

Reviewed-by: Christian Brauner <brauner@kernel.org>

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