[PATCH v2] block: Check ADMIN before NICE for IOPRIO_CLASS_RT

[Bart Van Assche]

On 11/15/21 10:16 AM, 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 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+
> ---
> 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
> +			 * 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;
>   			fallthrough;
>   			/* rt has prio field too */
> 

Are there any other SELinux policies (Fedora?) that need to be verified?

Anyway:

Reviewed-by: Bart Van Assche <bvanassche at acm.org>