Adds info about tag-based policies (#659)

Policies can now include conditions for certain types of actions. If the
object has an existing tag, the user can complete the action. Otherwise,
the user cannot.

In the Access Management doc
- Adds a section near the top about tag based policies
- In the conditional section, adds `s3.ExistingObjectTag` as an allowed
conditional for several specific actions.

Closes #604

Adds warning about weak policy condition keys.

Closes #596

source/administration/identity-access-management/policy-based-access-control.rst

@@ -13,21 +13,25 @@ Access Management
 Overview
 --------
 
-MinIO uses Policy-Based Access Control (PBAC) to define the authorized actions
-and resources to which an authenticated user has access. Each policy describes
-one or more :ref:`actions <minio-policy-actions>` and :ref:`conditions
-<minio-policy-conditions>` that outline the permissions of a 
-:ref:`user <minio-users>` or :ref:`group <minio-groups>` of
-users. 
+MinIO uses Policy-Based Access Control (PBAC) to define the authorized actions and resources to which an authenticated user has access. 
+Each policy describes one or more :ref:`actions <minio-policy-actions>` and :ref:`conditions <minio-policy-conditions>` that outline the permissions of a :ref:`user <minio-users>` or :ref:`group <minio-groups>` of users.
 
-MinIO PBAC is built for compatibility with AWS IAM policy syntax, structure, and
-behavior. The MinIO documentation makes a best-effort to cover IAM-specific
-behavior and functionality. Consider deferring to the :iam-docs:`IAM
-documentation <>` for more complete documentation on AWS IAM-specific topics.
+MinIO PBAC is built for compatibility with AWS IAM policy syntax, structure, and behavior. 
+The MinIO documentation makes a best-effort to cover IAM-specific behavior and functionality. 
+Consider deferring to the :iam-docs:`IAM documentation <>` for more complete documentation on AWS IAM-specific topics.
 
-The :mc:`mc admin policy` command supports creation and management of
-policies on the MinIO deployment. See the command reference for examples of
-usage.
+The :mc:`mc admin policy` command supports creation and management of policies on the MinIO deployment. 
+See the command reference for examples of usage.
+
+Tag-Based Policy Conditions
+---------------------------
+
+.. versionchanged:: RELEASE.2022-10-02T19-29-29Z
+
+   Policies can use conditions to limit a user's access only to objects with a specific tag.
+
+   MinIO supports :s3-docs:`tag-based conditionals <tagging-and-policies.html>` for policies for :ref:`selected actions <minio-selected-conditional-actions>`.
+   Use the ``s3:ExistingObjectTag/<key>`` in the ``Condition`` statement of the policy.
 
 .. _minio-policy-built-in:
 
@@ -263,6 +267,10 @@ subsections document actions for more advanced S3 operations:
    Controls access to the :s3-api:`GetObjectTagging <API_GetObjectTagging.html>`
    S3 API operation.
 
+.. policy-action:: s3:DeleteObjectTagging
+
+   Controls access to the :s3-api:`DeleteObjectTagging <API_DeleteObjectTagging.html>` S3 API operation.
+
 Bucket Configuration
 ~~~~~~~~~~~~~~~~~~~~
 
@@ -517,6 +525,15 @@ MinIO supports the following condition keys for all supported
 - ``aws:username``
 - ``x-amz-content-sha256``
 
+.. warning:: 
+
+   The ``aws:Referer``, ``aws:SourceIp``, and ``aws.UserAgent`` keys may be easily spoofed and therefore pose a potential security risk.
+   MinIO recommends only using these condition keys to *deny* access as a secondary security measure.
+   
+   **Never** use these three keys to grant access by themselves.
+
+.. _minio-selected-conditional-actions:
+
 The following table lists additional supported condition keys for specific
 actions:
 
@@ -531,6 +548,7 @@ actions:
    * - :policy-action:`s3:GetObject`
      - | ``x-amz-server-side-encryption``
        | ``x-amz-server-side-encryption-customer-algorithm``
+       | ``s3:ExistingObjectTag/<key>``
 
    * - :policy-action:`s3:ListBucket`
      - | ``prefix``
@@ -546,6 +564,7 @@ actions:
        | ``object-lock-retain-until-date``
        | ``object-lock-mode``
        | ``object-lock-legal-hold``
+       | ``s3:ExistingObjectTag/<key>``
 
    * - :policy-action:`s3:PutObjectRetention`
      - | ``x-amz-object-lock-remaining-retention-days``
@@ -564,10 +583,16 @@ actions:
    * - :policy-action:`s3:GetObjectVersion`
      - ``versionid``
 
-
    * - :policy-action:`s3:DeleteObjectVersion`
      - ``versionid``
 
+   * - :policy-action:`s3:PutObjectTagging`
+     - ``s3:ExistingObjectTag/<key>``
+
+   * - :policy-action:`s3:DeleteObjectTagging`
+     - ``s3:ExistingObjectTag/<key>``
+
+
 .. _minio-policy-mc-admin-actions:
 
 ``mc admin`` Policy Action Keys