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

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 <redacted>
Cc: Khazhismel Kumykov <redacted>
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+
---
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 */

-- 
2.34.0.rc1.387.gb447b232ab-goog

On 11/15/21 10:16 AM, Alistair Delva wrote:

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

Anyway:

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

On Mon, 15 Nov 2021 18:16:55 +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

[...]

Applied, thanks!

[1/1] block: Check ADMIN before NICE for IOPRIO_CLASS_RT
      commit: 94c4b4fd25e6c3763941bdec3ad54f2204afa992

Best regards,
-- 
Jens Axboe

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 <redacted>
Cc: Khazhismel Kumykov <redacted>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Serge Hallyn <serge@hallyn.com>

This won't harm anything, so

Acked-by: Serge Hallyn <serge@hallyn.com>

but questions below.

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