keys: Provide KEYCTL_GRANT_PERMISSION

Provide a keyctl() operation to grant/remove permissions.  The grant
operation, wrapped by libkeyutils, looks like:

	int ret = keyctl_grant_permission(key_serial_t key,
					  enum key_ace_subject_type type,
					  unsigned int subject,
					  unsigned int perm);

Where key is the key to be modified, type and subject represent the subject
to which permission is to be granted (or removed) and perm is the set of
permissions to be granted.  0 is returned on success.  SET_SECURITY
permission is required for this.

The subject type currently must be KEY_ACE_SUBJ_STANDARD for the moment
(other subject types will come along later).

For subject type KEY_ACE_SUBJ_STANDARD, the following subject values are
available:

	KEY_ACE_POSSESSOR	The possessor of the key
	KEY_ACE_OWNER		The owner of the key
	KEY_ACE_GROUP		The key's group
	KEY_ACE_EVERYONE	Everyone

perm lists the permissions to be granted:

	KEY_ACE_VIEW		Can view the key metadata
	KEY_ACE_READ		Can read the key content
	KEY_ACE_WRITE		Can update/modify the key content
	KEY_ACE_SEARCH		Can find the key by searching/requesting
	KEY_ACE_LINK		Can make a link to the key
	KEY_ACE_SET_SECURITY	Can set security
	KEY_ACE_INVAL		Can invalidate
	KEY_ACE_REVOKE		Can revoke
	KEY_ACE_JOIN		Can join this keyring
	KEY_ACE_CLEAR		Can clear this keyring

If an ACE already exists for the subject, then the permissions mask will be
overwritten; if perm is 0, it will be deleted.

Currently, the internal ACL is limited to a maximum of 16 entries.

For example:

	int ret = keyctl_grant_permission(key,
					  KEY_ACE_SUBJ_STANDARD,
					  KEY_ACE_OWNER,
					  KEY_ACE_VIEW | KEY_ACE_READ);

Signed-off-by: David Howells <dhowells@redhat.com>

security/keys/permission.c

@@ -278,3 +278,122 @@ long key_set_acl(struct key *key, struct key_acl *acl)
 	key_put_acl(acl);
 	return 0;
 }
+
+/*
+ * Allocate a new ACL with an extra ACE slot.
+ */
+static struct key_acl *key_alloc_acl(const struct key_acl *old_acl, int nr, int skip)
+{
+	struct key_acl *acl;
+	int nr_ace, i, j = 0;
+
+	nr_ace = old_acl->nr_ace + nr;
+	if (nr_ace > 16)
+		return ERR_PTR(-EINVAL);
+
+	acl = kzalloc(struct_size(acl, aces, nr_ace), GFP_KERNEL);
+	if (!acl)
+		return ERR_PTR(-ENOMEM);
+
+	refcount_set(&acl->usage, 1);
+	acl->nr_ace = nr_ace;
+	for (i = 0; i < old_acl->nr_ace; i++) {
+		if (i == skip)
+			continue;
+		acl->aces[j] = old_acl->aces[i];
+		j++;
+	}
+	return acl;
+}
+
+/*
+ * Generate the revised ACL.
+ */
+static long key_change_acl(struct key *key, struct key_ace *new_ace)
+{
+	struct key_acl *acl, *old;
+	int i;
+
+	old = rcu_dereference_protected(key->acl, lockdep_is_held(&key->sem));
+
+	for (i = 0; i < old->nr_ace; i++)
+		if (old->aces[i].type == new_ace->type &&
+		    old->aces[i].subject_id == new_ace->subject_id)
+			goto found_match;
+
+	if (new_ace->perm == 0)
+		return 0; /* No permissions to remove.  Add deny record? */
+
+	acl = key_alloc_acl(old, 1, -1);
+	if (IS_ERR(acl))
+		return PTR_ERR(acl);
+	acl->aces[i] = *new_ace;
+	goto change;
+
+found_match:
+	if (new_ace->perm == 0)
+		goto delete_ace;
+	if (new_ace->perm == old->aces[i].perm)
+		return 0;
+	acl = key_alloc_acl(old, 0, -1);
+	if (IS_ERR(acl))
+		return PTR_ERR(acl);
+	acl->aces[i].perm = new_ace->perm;
+	goto change;
+
+delete_ace:
+	acl = key_alloc_acl(old, -1, i);
+	if (IS_ERR(acl))
+		return PTR_ERR(acl);
+	goto change;
+
+change:
+	return key_set_acl(key, acl);
+}
+
+/*
+ * Add, alter or remove (if perm == 0) an ACE in a key's ACL.
+ */
+long keyctl_grant_permission(key_serial_t keyid,
+			     enum key_ace_subject_type type,
+			     unsigned int subject,
+			     unsigned int perm)
+{
+	struct key_ace new_ace;
+	struct key *key;
+	key_ref_t key_ref;
+	long ret;
+
+	new_ace.type = type;
+	new_ace.perm = perm;
+
+	switch (type) {
+	case KEY_ACE_SUBJ_STANDARD:
+		if (subject >= nr__key_ace_standard_subject)
+			return -ENOENT;
+		new_ace.subject_id = subject;
+		break;
+
+	default:
+		return -ENOENT;
+	}
+
+	key_ref = lookup_user_key(keyid, KEY_LOOKUP_PARTIAL, KEY_NEED_SETSEC);
+	if (IS_ERR(key_ref)) {
+		ret = PTR_ERR(key_ref);
+		goto error;
+	}
+
+	key = key_ref_to_ptr(key_ref);
+
+	down_write(&key->sem);
+
+	/* If we're not the sysadmin, we can only change a key that we own */
+	ret = -EACCES;
+	if (capable(CAP_SYS_ADMIN) || uid_eq(key->uid, current_fsuid()))
+		ret = key_change_acl(key, &new_ace);
+	up_write(&key->sem);
+	key_put(key);
+error:
+	return ret;
+}