Update information as a partial response to the post

  From: "Chris D. Peterson" <cpeterson@aventail.com>
  Subject: Implementation Issues with OpenSSL
  To: openssl-users@openssl.org
  Date: Wed, 22 Aug 2001 16:13:17 -0700
The patch included in the original post may improve the internal session
list handling (and is therefore worth a seperate investigation).
No change to the list handling will however solve the problems of incorrect
SSL_SESSION_free() calls. The session list is only one possible point of
failure, dangling pointers would also occur for SSL object currently
using the session. The correct solution is to only use SSL_SESSION_free()
when applicable!

1 files changed, 29 insertions, 1 deletions

diff --git a/doc/ssl/SSL_SESSION_free.pod b/doc/ssl/SSL_SESSION_free.pod
index 9275d88a40..558de01df9 100644
--- a/doc/ssl/SSL_SESSION_free.pod
+++ b/doc/ssl/SSL_SESSION_free.pod
@@ -16,12 +16,40 @@ SSL_SESSION_free() decrements the reference count of B<session> and removes
 the B<SSL_SESSION> structure pointed to by B<session> and frees up the allocated
 memory, if the the reference count has reached 0.
 
+=head1 NOTES
+
+SSL_SESSION objects are allocated, when a TLS/SSL handshake operation
+is successfully completed. Depending on the settings, see
+L<SSL_CTX_set_session_cache_mode(3)|SSL_CTX_set_session_cache_mode(3)>,
+the SSL_SESSION objects are internally referenced by the SSL_CTX and
+linked into its session cache. SSL objects may be using the SSL_SESSION object;
+as a session may be reused, several SSL objects may be using one SSL_SESSION
+object at the same time. It is therefore crucial to keep the reference
+count (usage information) correct and not delete a SSL_SESSION object
+that is still used, as this may lead to program failures due to
+dangling pointers. These failures may also appear delayed, e.g.
+when an SSL_SESSION object was completely freed as the reference count
+incorrectly became 0, but it is still referenced in the internal
+session cache and the cache list is processed during a
+L<SSL_CTX_flush_sessions(3)|SSL_CTX_flush_sessions(3)> operation.
+
+SSL_SESSION_free() must only be called for SSL_SESSION objects, for
+which the reference count was explicitly incremented (e.g.
+by calling SSL_get1_session(), see L<SSL_get_session(3)|SSL_get_session(3)>)
+or when the SSL_SESSION object was generated outside a TLS handshake
+operation, e.g. by using L<d2i_SSL_SESSION(3)|d2i_SSL_SESSION(3)>.
+It must not be called on other SSL_SESSION objects, as this would cause
+incorrect reference counts and therefore program failures.
+
 =head1 RETURN VALUES
 
 SSL_SESSION_free() does not provide diagnostic information.
 
 =head1 SEE ALSO
 
-L<ssl(3)|ssl(3)>, L<SSL_get_session(3)|SSL_get_session(3)>
+L<ssl(3)|ssl(3)>, L<SSL_get_session(3)|SSL_get_session(3)>,
+L<SSL_CTX_set_session_cache_mode(3)|SSL_CTX_set_session_cache_mode(3)>,
+L<SSL_CTX_flush_sessions(3)|SSL_CTX_flush_sessions(3)>,
+ L<d2i_SSL_SESSION(3)|d2i_SSL_SESSION(3)>
 
 =cut