[PATCH] include/uapi/linux/lsm.h, Documentation/userspace-api/lsm.rst: introduce LSM_ATTR_UNSHARE

[Stephen Smalley]

On Fri, Oct 3, 2025 at 9:23 AM Stephen Smalley
<stephen.smalley.work at gmail.com> wrote:
>
> This defines a new LSM_ATTR_UNSHARE attribute for the
> lsm_set_self_attr(2) and lsm_get_self_attr(2) system calls. When
> passed to lsm_set_self_attr(2), the LSM-specific namespace for the
> specified LSM id is immediately unshared in a similar manner to the
> unshare(2) system call for other Linux namespaces. When passed to
> lsm_get_self_attr(2), the return value is a boolean (0 or 1) that
> indicates whether the LSM-specific namespace for the specified LSM id
> has been unshared and not yet fully initialized (e.g. no policy yet
> loaded).

Upon implementing the 2nd part for SELinux, it turns out that the
lsm_get_self_attr(2) call can't unambiguously return 0 or 1 due to the
current interface definition, so will be spinning a v2 that instead
sets the *size argument accordingly.

>
> Link: https://lore.kernel.org/selinux/20250918135904.9997-2-stephen.smalley.work@gmail.com/
> Link: https://lore.kernel.org/selinux/CAHC9VhRGMmhxbajwQNfGFy+ZFF1uN=UEBjqQZQ4UBy7yds3eVQ@mail.gmail.com/
>
> Signed-off-by: Stephen Smalley <stephen.smalley.work at gmail.com>
> ---
>  Documentation/userspace-api/lsm.rst | 9 +++++++++
>  include/uapi/linux/lsm.h            | 1 +
>  2 files changed, 10 insertions(+)
>
> diff --git a/Documentation/userspace-api/lsm.rst b/Documentation/userspace-api/lsm.rst
> index a76da373841b..93638c1e275a 100644
> --- a/Documentation/userspace-api/lsm.rst
> +++ b/Documentation/userspace-api/lsm.rst
> @@ -48,6 +48,15 @@ creating socket objects.
>  The proc filesystem provides this value in ``/proc/self/attr/sockcreate``.
>  This is supported by the SELinux security module.
>
> +``LSM_ATTR_UNSHARE`` is used to unshare the LSM-specific namespace for
> +the process. When passed to ``lsm_set_self_attr(2)``, the LSM-specific
> +namespace for the specified LSM id is immediately unshared
> +in a similar manner to the ``unshare(2)`` system call for other
> +Linux namespaces. When passed to ``lsm_get_self_attr(2)``,
> +the return value is a boolean (0 or 1) that indicates whether the
> +LSM-specific namespace for the specified LSM id has been unshared
> +and not yet fully initialized (e.g. no policy yet loaded).
> +
>  Kernel interface
>  ================
>
> diff --git a/include/uapi/linux/lsm.h b/include/uapi/linux/lsm.h
> index 938593dfd5da..fb1b4a8aa639 100644
> --- a/include/uapi/linux/lsm.h
> +++ b/include/uapi/linux/lsm.h
> @@ -83,6 +83,7 @@ struct lsm_ctx {
>  #define LSM_ATTR_KEYCREATE     103
>  #define LSM_ATTR_PREV          104
>  #define LSM_ATTR_SOCKCREATE    105
> +#define LSM_ATTR_UNSHARE       106
>
>  /*
>   * LSM_FLAG_XXX definitions identify special handling instructions
> --
> 2.51.0
>