I was installing Standalone cluster Oracle 12c Installation on VM RHEL 6 .

I was following every instruction and i was getting struck for permission on asm disk .After reboot permission of the disk changes

After long search and researching i resolved this as follows

step 1) Oracle or grid user need to be a part of group OSASM it could be dba or asmdba or asmadmin anything

step 2) fdisk -l | grep dev :-- It will reflect all the disk

For Ex

[root@str1 dev]# fdisk -l | grep dev
Disk /dev/sda: 134.2 GB, 134217728000 bytes
/dev/sda1   *           1        3917    31457280   83  Linux
/dev/sda2            3917        9108    41697280   83  Linux
/dev/sda3            9108       10413    10485760   83  Linux
/dev/sda4           10413       16318    47430656    5  Extended
/dev/sda5           10413       11719    10485760   83  Linux
/dev/sda6           11719       13024    10485760   82  Linux swap / Solaris
/dev/sda7           13025       16318    26456064   83  Linux
Disk /dev/sdb: 10.7 GB, 10737418240 bytes
/dev/sdb1               1        1305    10482381   83  Linux
Disk /dev/sdc: 10.7 GB, 10737418240 bytes
/dev/sdc1               1        1305    10482381   83  Linux
[root@str1 dev]#


Step 3) /dev/sdc , /dev/sdc1 , /dev/sdb & /dev/sdb1 are my candidate disk for asm

step 4) partprobe /dev/sdc
step 5) partprobe /dev/sdc1
step 6) partprobe /dev/sdb
step 7) partprobe /dev/sdb1
step 8) vi /etc/udev/rules.d/99-oracle-asmdevices.rules
            KERNEL=="sdc1", NAME="asmdisk1", OWNER="oracle", GROUP="asmadmin", MODE="0660"
KERNEL=="sdb1", NAME="asmdisk2", OWNER="oracle", GROUP="asmadmin", MODE="0660"

step 9) udevadm control --reload-rules
step 10) start_udev
step 11) cd /dev
step 12) [root@str1 dev]# ls -ltr asm*
brw-rw----. 1 oracle asmadmin 8, 17 Apr  3 17:49 asmdisk2
brw-rw----. 1 oracle asmadmin 8, 33 Apr  3 17:49 asmdisk1

By following above steps my asm issue got resolved ..