The following commit, which has introduced a new feature for PostgreSQL 11, introduces the possibility to lower a bit the set of permissions around data folders:
commit: c37b3d08ca6873f9d4eaf24c72a90a550970cbb8 author: Stephen Frost <firstname.lastname@example.org> date: Sat, 7 Apr 2018 17:45:39 -0400 Allow group access on PGDATA Allow the cluster to be optionally init'd with read access for the group. This means a relatively non-privileged user can perform a backup of the cluster without requiring write privileges, which enhances security. The mode of PGDATA is used to determine whether group permissions are enabled for directory and file creates. This method was chosen as it's simple and works well for the various utilities that write into PGDATA. Changing the mode of PGDATA manually will not automatically change the mode of all the files contained therein. If the user would like to enable group access on an existing cluster then changing the mode of all the existing files will be required. Note that pg_upgrade will automatically change the mode of all migrated files if the new cluster is init'd with the -g option. Tests are included for the backend and all the utilities which operate on the PG data directory to ensure that the correct mode is set based on the data directory permissions. Author: David Steele <email@example.com> Reviewed-By: Michael Paquier, with discussion amongst many others. Discussion: https://postgr.es/m/ad346fe6-b23e-59f1-ecb7-0e08390ad629%40pgmasters.net
Group access on the data folder means that files can optionally use 0640 as mask and folders can use 0750, which becomes handy for particularly backup scenarios where a different user than the one running PostgreSQL would be sufficient to take a backup of the instance. For some security policies, it is important to do an operation with a user which has the minimum set of permissions allowing to perform the task, so in this case a user which is member of the same group as the one running the PostgreSQL instance would be able to read all files in a data folder and take a backup from it. So not only this is useful for people implementing their own backup tool, but also for administrators looking at users able to do the backup task with only a minimal set of access permissions.
The feature can be enabled using initdb -g/–allow-group-access, which will create files using 0640 as mask and folder using 0750. Note that in v10 and older versions, trying to start a server with the base data folder having a permission different than 0700 results in a failure of the postmaster process, so with v11 and above the postmaster is able to start if the data folder is found as using either 0700 or 0750. Note that an administrator can also perfectly initialize a data folder without the option –allow-group-access first, and change it to use group permissions after with chmod -R or such, and the cluster will adapt automatically. In order to know if a data folder uses group access, a new GUC parameter called data_directory_mode is available, which returns the mask used, so for a data folder allowing group access you would see that:
=# SHOW data_directory_mode; data_directory_mode --------------------- 0750 (1 row)
Sometimes deployments of PostgreSQL use advanced backup strategies mixing multiple solutions, which is why the following in-core tools also respect if group access is allowed in a cluster when fetching and writing files related to the cluster:
- pg_basebackup, which respects permissions for both the tar and plain formats.
- pg_receivewal will create new WAL segments using group permissions.
- pg_recvlogical does the same for logical changes received.
Note that it is not possible to enforce the mask received, so if a cluster has group access enabled, then all the tools mentioned above will automatically switch to it. It is not possible to write data with group access when the data folder does not use it, as well as to write data without group access when the data folder uses group access. So all the behaviors are kept consistent for simplicity.
For developers of tools and plugins in charge of writing data for a data folder or anything related to PostgreSQL, there is a simple way to track if group access is enabled on an instance. First, if you use a normal libpq connection, it is possible to check after data_directory_mode using the SHOW command (works as well with the replication protocol!). For tools working directly on a data folder, like pg_rewind or pg_resetwal, there is a new API available called GetDataDirectoryCreatePerm() which can be used to set a couple of low-level variables which would set the mask needed for files and folders automatically:
- pg_mode_mask for the mode mask, usable with umask().
- pg_file_create_mode, for file creation mask.
- pg_dir_create_mode, for directory creation mask.
So you may want to patch your tool so as this is made extensible in a way consistent with PostgreSQL 11 or newer versions.
One last thing. Be careful of SSL certificates or such in the data folder when allowing group access as it could result in errors with the software doing the backup. Fortunately those can be located outside the data folder.