[PATCH 02/10] capability: add any wrappers to test for multiple caps with exactly one audit message

[Andrii Nakryiko]

On Fri, Mar 15, 2024 at 4:39 AM Christian Göttsche
<cgzones at googlemail.com> wrote:
>
> Add the interfaces `capable_any()` and `ns_capable_any()` as an
> alternative to multiple `capable()`/`ns_capable()` calls, like
> `capable_any(CAP_SYS_NICE, CAP_SYS_ADMIN)` instead of
> `capable(CAP_SYS_NICE) || capable(CAP_SYS_ADMIN)`.
>
> `capable_any()`/`ns_capable_any()` will in particular generate exactly
> one audit message, either for the left most capability in effect or, if
> the task has none, the first one.
>
> This is especially helpful with regard to SELinux, where each audit
> message about a not allowed capability request will create a denial
> message.  Using this new wrapper with the least invasive capability as
> left most argument (e.g. CAP_SYS_NICE before CAP_SYS_ADMIN) enables
> policy writers to only grant the least invasive one for the particular
> subject instead of both.
>
> CC: linux-block at vger.kernel.org
> Signed-off-by: Christian Göttsche <cgzones at googlemail.com>
> ---
> v5:
>    - add check for identical passed capabilities
>    - rename internal helper according to flag rename to
>      ns_capable_noauditondeny()
> v4:
>    Use CAP_OPT_NODENYAUDIT via added ns_capable_nodenyaudit()
> v3:
>    - rename to capable_any()
>    - fix typo in function documentation
>    - add ns_capable_any()
> v2:
>    avoid varargs and fix to two capabilities; capable_or3() can be added
>    later if needed
> ---
>  include/linux/capability.h | 10 ++++++
>  kernel/capability.c        | 73 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 83 insertions(+)
>

[...]

>
> +/**
> + * ns_capable_any - Determine if the current task has one of two superior capabilities in effect
> + * @ns:  The usernamespace we want the capability in
> + * @cap1: The capabilities to be tested for first
> + * @cap2: The capabilities to be tested for secondly
> + *
> + * Return true if the current task has at least one of the two given superior
> + * capabilities currently available for use, false if not.
> + *
> + * In contrast to or'ing capable() this call will create exactly one audit
> + * message, either for @cap1, if it is granted or both are not permitted,
> + * or @cap2, if it is granted while the other one is not.
> + *
> + * The capabilities should be ordered from least to most invasive, i.e. CAP_SYS_ADMIN last.
> + *
> + * This sets PF_SUPERPRIV on the task if the capability is available on the
> + * assumption that it's about to be used.
> + */
> +bool ns_capable_any(struct user_namespace *ns, int cap1, int cap2)
> +{
> +       if (cap1 == cap2)
> +               return ns_capable(ns, cap1);
> +
> +       if (ns_capable_noauditondeny(ns, cap1))
> +               return true;
> +
> +       if (ns_capable_noauditondeny(ns, cap2))
> +               return true;
> +
> +       return ns_capable(ns, cap1);

this will incur an extra capable() check (with all the LSMs involved,
etc), and so for some cases where capability is expected to not be
present, this will be a regression. Is there some way to not redo the
check, but just audit the failure? At this point we do know that cap1
failed before, so might as well just log that.

> +}
> +EXPORT_SYMBOL(ns_capable_any);
> +
> +/**
> + * capable_any - Determine if the current task has one of two superior capabilities in effect
> + * @cap1: The capabilities to be tested for first
> + * @cap2: The capabilities to be tested for secondly
> + *
> + * Return true if the current task has at least one of the two given superior
> + * capabilities currently available for use, false if not.
> + *
> + * In contrast to or'ing capable() this call will create exactly one audit
> + * message, either for @cap1, if it is granted or both are not permitted,
> + * or @cap2, if it is granted while the other one is not.
> + *
> + * The capabilities should be ordered from least to most invasive, i.e. CAP_SYS_ADMIN last.
> + *
> + * This sets PF_SUPERPRIV on the task if the capability is available on the
> + * assumption that it's about to be used.
> + */
> +bool capable_any(int cap1, int cap2)
> +{
> +       return ns_capable_any(&init_user_ns, cap1, cap2);
> +}
> +EXPORT_SYMBOL(capable_any);
> +
>  /**
>   * capable - Determine if the current task has a superior capability in effect
>   * @cap: The capability to be tested for
> --
> 2.43.0
>
>