Description of problem:

Sbd daemon is not allowed to write/append to the /proc/sysrq-trigger for reboot/off/panic action.

Version-Release number of selected component (if applicable):
# rpm -qa selinux-policy*
selinux-policy-targeted-3.14.1-51.el8.noarch
selinux-policy-3.14.1-51.el8.noarch

How reproducible:
always

Steps to Reproduce:

1. install sbd package

# yum -y -q install sbd

2. create loop device:

# dd if=/dev/zero of=part-00 bs=1M count=400
# losetup /dev/loop0 part-00

3. create sbd heder on the loop device:

# sbd -d /dev/loop0 create
Initializing device /dev/loop0
Creating version 2.1 header on device 4 (uuid: db84b618-fb0b-4631-b6bc-54f3664d00a1)
Initializing 255 slots on device 4
Device /dev/loop0 is initialized.


4. prepare sbd configuration file

# cat > /etc/sysconfig/sbd <<EOF
# enable debug mode, which does panic instead of reboot
SBD_OPTS="-Z"
SBD_DELAY_START=no
SBD_DEVICE="/dev/loop0"
SBD_PACEMAKER=yes
SBD_STARTMODE=always
SBD_WATCHDOG_DEV=/dev/null
SBD_WATCHDOG_TIMEOUT=5
EOF

5. Modify sbd.service file to enable start service manualy:

# sed -i "s/^RefuseManualStart=.*$/RefuseManualStart=false/" /usr/lib/systemd/system/sbd.service
# sed -i "s/^RefuseManualStop=.*$/RefuseManualStop=false/" /usr/lib/systemd/system/sbd.service
# systemctl daemon-reload

6. Start the sbd service:

# systemctl start sbd
# systemctl show sbd -p ActiveState
ActiveState=active

7. Write 'reset' message to the slot on the sbd device (use sbd's list command to get the slot name).

# sbd -d /dev/loop0 message $(sbd -d /dev/loop0 list | awk '{print $2}') reset

Broadcast message from systemd-journald.lab.eng.brq.redhat.com (Wed 2019-01-30 20:39:16 CET):

sbd[17401]:    emerg: do_exit: Rebooting system: crashdump


8. check the avc messages

# getenforce
Enforcing
# ausearch -m avc -m user_avc -m selinux_err -m user_selinux_err -i
----
type=PROCTITLE msg=audit(01/30/2019 20:39:16.015:1576) : proctitle=sbd: inquisitor 
type=SYSCALL msg=audit(01/30/2019 20:39:16.015:1576) : arch=x86_64 syscall=openat success=no exit=EACCES(Permission denied) a0=0xffffff9c a1=0x5652549be7ad a2=O_WRONLY|O_CREAT|O_APPEND a3=0x1b6 items=0 ppid=1 pid=17401 auid=unset uid=root gid=root euid=root suid=root fsuid=root egid=root sgid=root fsgid=root tty=(none) ses=unset comm=sbd exe=/usr/sbin/sbd subj=system_u:system_r:sbd_t:s0 key=(null) 
type=AVC msg=audit(01/30/2019 20:39:16.015:1576) : avc:  denied  { append } for  pid=17401 comm=sbd name=sysrq-trigger dev="proc" ino=4026532092 scontext=system_u:system_r:sbd_t:s0 tcontext=system_u:object_r:sysctl_t:s0 tclass=file permissive=0


Actual results:

sbd daemon is not allowed to write/append to the /proc/sysrq-trigger

Expected results:

sbd daemon is allowed to write/append to the /proc/sysrq-trigger

----------------
Additional info:

The same issue was in bz1455631 where apppend/sys_boot were denied.

a 'sys_boot' issue was solved by

allow sbd_t sbd_t : capability { sys_boot }

but

allow sbd_t proc_t : dir { write }

did not help to allow append to /proc/sysrq-trigger which has different context.

# ls -lZ /proc/sysrq-trigger 
--w-------. 1 root root system_u:object_r:sysctl_t:s0 0 Jan 24 09:52 /proc/sysrq-trigger