On Mon, Nov 15, 2021 at 06:16:55PM +0000, Alistair Delva 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.
>
> Fixes: 9d3a39a5f1e4 ("block: grant IOPRIO_CLASS_RT to CAP_SYS_NICE")
> Signed-off-by: Alistair Delva <adelva@xxxxxxxxxx>
> Cc: Khazhismel Kumykov <khazhy@xxxxxxxxxx>
> Cc: Bart Van Assche <bvanassche@xxxxxxx>
> Cc: Serge Hallyn <serge@xxxxxxxxxx>
This won't harm anything, so
Acked-by: Serge Hallyn <serge@xxxxxxxxxx>
but questions below.
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
> Cc: selinux@xxxxxxxxxxxxxxx
> Cc: linux-security-module@xxxxxxxxxxxxxxx
> Cc: kernel-team@xxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx # v5.14+
> ---
> v2: added comment requested by Jens
> block/ioprio.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/block/ioprio.c b/block/ioprio.c
> index 0e4ff245f2bf..313c14a70bbd 100644
> --- a/block/ioprio.c
> +++ b/block/ioprio.c
> @@ -69,7 +69,14 @@ int ioprio_check_cap(int ioprio)
>
> switch (class) {
> case IOPRIO_CLASS_RT:
> - if (!capable(CAP_SYS_NICE) && !capable(CAP_SYS_ADMIN))
> + /*
> + * Originally this only checked for CAP_SYS_ADMIN,
> + * which was implicitly allowed for pid 0 by security
What do you mean, implicitly allowed for pid 0? Can you point to where
that happens?
> + * modules such as SELinux. Make sure we check
> + * CAP_SYS_ADMIN first to avoid a denial/avc for
> + * possibly missing CAP_SYS_NICE permission.
> + */
> + if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_NICE))
> return -EPERM;
But whichever one comes first can cause an avc denial message. It seems
like we need a new capable() primitive which supports multiple bits,
when more than one can authorize an action, and which emits an audit
message only if all bits are missing.
> fallthrough;
> /* rt has prio field too */
> --
> 2.34.0.rc1.387.gb447b232ab-goog
