What is Oracle ASM File Access Control and Why You Need It

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What is Oracle ASM File Access Control

Oracle ASM File Access Control restricts the access of files to specific Oracle ASM clients (mainly database)that connect as SYSDBA.
Enabling ASM File Access Control requires following
• Linux or UNIX operating system.
• Job role separation at the OS level
• Disk group attributes must be set:
 – COMPATIBLE.ASM to 11.2 or higher
– COMPATIBLE.RDBMS to 11.2 or higher
– ACCESS_CONTROL.ENABLED to TRUE
– ACCESS_CONTROL.UMASK to a mask value

What ACLs allows: 

• Set permissions at the ASM file level.
• Permissions are none (0), read (4), or read-write (6).
• Permissions are available only on Linux and UNIX operating systems.

When ASM File Access Control is enabled and an ASM file is created the file permissions is set as per the current access_control.umask setting of the DG
default value for which is 066
The permission of an ASM file can be granted to
owner,   group,   other
{0|2|6}  {0|2|6}  {0|2|6}
0 masks out nothing
2 masks out write
6 masks out both read and write permissions.

Use Case:
I have 2 databases configured as follows
DB_NAME         OS_USER       DG_GROUP
TST1T                dbatst1             DG_TST_DATA
TST2T                dbatst2             DG_TST2_DATA

Check the Current Status of Access Control for DG 

SQL> select GROUP_NUMBER,name from v$asm_diskgroup where name='DG_TST2_DATA';

GROUP_NUMBER NAME
------------ ------------------------------
           1 DG_TST2_DATA

SQL> select NAME,VALUE,GROUP_NUMBER from  v$asm_attribute where GROUP_NUMBER=1 and NAME like '%access_control%';

NAME                           VALUE                                    GROUP_NUMBER
------------------------------ ---------------------------------------- ------------
access_control.enabled         false                                               1
access_control.umask           066                                                 1

Access Control for DG DG_TST2_DATA is false

Create a Dummy Tablespace

Lets us create a tablespace in dbatst1 database in DG_TST2_DATA DG
SQL> create tablespace AFAC_TST datafile '+DG_TST2_DATA' size 10M;

Tablespace created.
SQL> select name from v$datafile where name like '%_TST2%';

NAME
--------------------------------------------------------------------------------
+DG_TST2_DATA/TST1T/DATAFILE/afac_tst.269.987345611

Lets login to ASM and try to delete this data file

login to OS as dbatst2 (which is not the owner of TST1T database) and login to ASM instance as sysdba

[dbatst2@TSTBOX]$asmcmd --privilege sysdba
ASMCMD>
ASMCMD> ls -l +DG_TST2_DATA/TST1T/DATAFILE/afac_tst.269.987345611
Type      Redund  Striped  Time             Sys  Name
DATAFILE  UNPROT  COARSE   SEP 20 14:00:00  Y    afac_tst.269.987345611

ASMCMD> rm AFAC_TST.269.987345611
ORA-15032: not all alterations performed
ORA-15028: ASM file '+DG_TST2_DATA/TST1T/DATAFILE/AFAC_TST.269.987345611' not dropped; currently being accessed (DBD ERROR: OCIStmtExecute)

But What if the Database is Down

SQL> select name from v$database;

NAME
---------
TST1T

SQL> shut immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.

File Got Deleted.............
So if you are using separation of duties at OS Level you will definitely don't wont any third OS user (non DB Owner OS User) to delete your files from ASM.
And yes you can achieve this using oracle ASM file access control.

Now let us enable the ASM file access control and repeat the same again

How to Enable ASM File Access Control

Login to ASM and enable file access control

SQL>ALTER DISKGROUP DG_TST2_DATA SET ATTRIBUTE 'access_control.enabled' = 'true';

Create a Dummy Tablespace

Lets us create a tablespace in dbatst1 database in DG_TST2_DATA DG

SQL> create tablespace AFAC_TST datafile '+DG_TST2_DATA' size 10M;

Tablespace created.
SQL> select name from v$datafile where name like '%_TST2%';

NAME
--------------------------------------------------------------------------------
+DG_TST2_DATA/TST1T/DATAFILE/afac_tst.269.987348441

Lets login to ASM and try to delete this data file
login to OS as dbatst2 (which is not the owner of TST1T database) and login to ASM instance as sysdba

[dbatst2@TSTBOX]$asmcmd --privilege sysdba
ASMCMD> ls -l +DG_TST2_DATA/TST1T/DATAFILE/afac_tst.269.987348441
Type      Redund  Striped  Time             Sys  Name
DATAFILE  UNPROT  COARSE   SEP 20 15:00:00  Y    afac_tst.269.987348441
ASMCMD> cd +DG_TST2_DATA/TST1T/DATAFILE
ASMCMD> ls
AFAC_TST.269.987348441
ASMCMD> rm AFAC_TST.269.987348441
ORA-15032: not all alterations performed
ORA-15260: permission denied on ASM disk group (DBD ERROR: OCIStmtExecute)

Conclusion:- Oracle ASM File access control feature is really very useful when you are running shared infrastructure and you  want to avoid any accidental damage/deletion of the file alias etc..