add a caveat on access control issues

diff --git a/doc/man/man5/slapo-pcache.5 b/doc/man/man5/slapo-pcache.5
index ff977f7a7d3557b75b74e7540bdd2e20a9104ff9..a0e1bb6370a25353a158b83541da4cc6829d6edc 100644 (file)
--- a/doc/man/man5/slapo-pcache.5
+++ b/doc/man/man5/slapo-pcache.5
@@ -133,7 +133,52 @@ cachesize 100
 .RE
 .LP
 Any valid directives for the chosen database type may be used.
+.SH CAVEATS
+Caching data is prone to inconsistencies because updates on the remote server
+will not be reflected in the response of the cache at least (and at most)
+for the duration of the
+.B proxytemplate
+.BR TTL .
+
+Another potential (and subtle) inconsistency may occur when data is retrieved 
+with different identities and specific per-identity access control
+is enforced by the remote server.
+If data was retrieved with an identity that collected only partial results
+because of access rules enforcement on the remote server, other users
+with different access privileges on the remote server will get different
+results from the remote server and from the cache.
+If those users have higher access privileges on the remote server, they will 
+get from the cache only a subset of the results they would get directly 
+from the remote server; but if they have lower access privileges, they will 
+get from the cache a superset of the results they would get directly 
+from the remote server.
+Either occurrence may or may not be acceptable, based on the security policy
+of the cache and of the remote server.
+It is important to note that in this case the proxy is violating the security
+of the remote server by disclosing to an identity data that was collected 
+by another identity.
+For this reason, it is suggested that, when using
+.BR back-ldap ,
+proxy caching be used in conjunction with the 
+.I identity assertion
+feature of
+.BR slapd-ldap (5)
+(see the
+.B idassert-bind
+and the
+.B idassert-authz
+statements), so that remote server interrogation occurs with a vanilla identity 
+that has some relatively high
+.B search
+and
+.B read
+access privileges, and the "real" access control is delegated to the proxy's ACLs.
+Beware that since only the cached fraction of the real datum is available
+to the cache, it may not be possible to enforce the same access rules that
+are defined on the remote server.
+When security is a concern, cached proxy access must be carefully tailored.
 .SH FILES
+
 .TP
 ETCDIR/slapd.conf
 default slapd configuration file