[PATCH] block: Check ADMIN before NICE for IOPRIO_CLASS_RT

[Ondrej Mosnacek]

On Mon, Nov 15, 2021 at 7:14 PM Alistair Delva <adelva at google.com> wrote:
> Booting to Android userspace on 5.14 or newer triggers the following
> SELinux denial:
>
> avc: denied { sys_nice } for comm="init" capability=23
>      scontext=u:r:init:s0 tcontext=u:r:init:s0 tclass=capability
>      permissive=0
>
> Init is PID 0 running as root, so it already has CAP_SYS_ADMIN. For
> better compatibility with older SEPolicy, check ADMIN before NICE.

But with this patch you in turn punish the new/better policies that
try to avoid giving domains CAP_SYS_ADMIN unless necessary (using only
the more granular capabilities wherever possible), which may now get a
bogus sys_admin denial. IMHO the order is better as it is, as it
motivates the "good" policy writing behavior - i.e. spelling out the
capability permissions more explicitly and avoiding CAP_SYS_ADMIN.

IOW, if you domain does CAP_SYS_NICE things, and you didn't explicitly
grant it that (and instead rely on the CAP_SYS_ADMIN fallback), then
the denial correctly flags it as an issue in your policy and
encourages you to add that sys_nice permission to the domain. Then
when one beautiful hypothetical day the CAP_SYS_ADMIN fallback is
removed, your policy will be ready for that and things will keep
working.

Feel free to carry that patch downstream if patching the kernel is
easier for you than fixing the policy, but for the upstream kernel
this is just a step in the wrong direction.

>
> Fixes: 9d3a39a5f1e4 ("block: grant IOPRIO_CLASS_RT to CAP_SYS_NICE")
> Signed-off-by: Alistair Delva <adelva at google.com>
> Cc: Khazhismel Kumykov <khazhy at google.com>
> Cc: Bart Van Assche <bvanassche at acm.org>
> Cc: Serge Hallyn <serge at hallyn.com>
> Cc: Jens Axboe <axboe at kernel.dk>
> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> Cc: Paul Moore <paul at paul-moore.com>
> Cc: selinux at vger.kernel.org
> Cc: linux-security-module at vger.kernel.org
> Cc: kernel-team at android.com
> Cc: stable at vger.kernel.org # v5.14+
> ---
>  block/ioprio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/ioprio.c b/block/ioprio.c
> index 0e4ff245f2bf..4d59c559e057 100644
> --- a/block/ioprio.c
> +++ b/block/ioprio.c
> @@ -69,7 +69,7 @@ int ioprio_check_cap(int ioprio)
>
>         switch (class) {
>                 case IOPRIO_CLASS_RT:
> -                       if (!capable(CAP_SYS_NICE) && !capable(CAP_SYS_ADMIN))
> +                       if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_NICE))
>                                 return -EPERM;
>                         fallthrough;
>                         /* rt has prio field too */
> --
> 2.34.0.rc1.387.gb447b232ab-goog
>

--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.