block: Check ADMIN before NICE for IOPRIO_CLASS_RT

BugLink: https://bugs.launchpad.net/bugs/1952579
commit 94c4b4fd25e6c3763941bdec3ad54f2204afa992 upstream.

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@google.com>
Cc: Khazhismel Kumykov <khazhy@google.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Paul Moore <paul@paul-moore.com>
Cc: selinux@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Cc: kernel-team@android.com
Cc: stable@vger.kernel.org # v5.14+
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Serge Hallyn <serge@hallyn.com>
Link: https://lore.kernel.org/r/20211115181655.3608659-1-adelva@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

diff --git a/block/ioprio.c b/block/ioprio.c
index 0e4ff245f2bf21b7c1ab7d5b36ac23b89decf7a0..313c14a70bbd3985bd96a81767b0ebb776b3e341 100644 (file)
--- 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 */