linux-kernel - [PATCH v2] block: Check ADMIN before NICE for IOPRIO_CLASS

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Date:   Mon, 15 Nov 2021 18:16:55 +0000
From:   Alistair Delva <adelva@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     Khazhismel Kumykov <khazhy@...gle.com>,
        Bart Van Assche <bvanassche@....org>,
        Serge Hallyn <serge@...lyn.com>, Jens Axboe <axboe@...nel.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Paul Moore <paul@...l-moore.com>, selinux@...r.kernel.org,
        linux-security-module@...r.kernel.org, kernel-team@...roid.com,
        stable@...r.kernel.org
Subject: [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 <adelva@...gle.com>
Cc: Khazhismel Kumykov <khazhy@...gle.com>
Cc: Bart Van Assche <bvanassche@....org>
Cc: Serge Hallyn <serge@...lyn.com>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Paul Moore <paul@...l-moore.com>
Cc: selinux@...r.kernel.org
Cc: linux-security-module@...r.kernel.org
Cc: kernel-team@...roid.com
Cc: stable@...r.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