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!

doc/ssl/SSL_CTX_sess_set_get_cb.pod

@@ -70,12 +70,16 @@ proposed by the client. The get_session_cb() is always called, also when
 session caching was disabled. The get_session_cb() is passed the
 B<ssl> connection, the session id of length B<length> at the memory location
 B<data>. With the parameter B<copy> the callback can require the
-SSL engine to increment the reference count of the SSL_SESSION object.
+SSL engine to increment the reference count of the SSL_SESSION object,
+Normally the reference count is not incremented and therefore the
+session must not be explicitly freed with
+L<SSL_SESSION_free(3)|SSL_SESSION_free(3)>.
 
 =head1 SEE ALSO
 
 L<ssl(3)|ssl(3)>, L<d2i_SSL_SESSION(3)|d2i_SSL_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<SSL_CTX_flush_sessions(3)|<SSL_CTX_flush_sessions(3)>,
+L<SSL_SESSION_free(3)|SSL_SESSION_free(3)>
 
 =cut

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

doc/ssl/SSL_get_session.pod

@@ -40,6 +40,12 @@ If the data is to be kept, SSL_get1_session() will increment the reference
 count and the session will stay in memory until explicitly freed with
 L<SSL_SESSION_free(3)|SSL_SESSION_free(3)>, regardless of its state.
 
+SSL_SESSION objects keep internal link information about the session cache
+list, when being inserted into one SSL_CTX object's session cache.
+One SSL_SESSION object, regardless of its reference count, must therefore
+only be used with one SSL_CTX object (and the SSL objects created
+from this SSL_CTX object).
+
 =head1 RETURN VALUES
 
 The following return values can occur:

doc/ssl/SSL_set_session.pod

@@ -23,6 +23,14 @@ If there is already a session set inside B<ssl> (because it was set with
 SSL_set_session() before or because the same B<ssl> was already used for
 a connection), SSL_SESSION_free() will be called for that session.
 
+=head1 NOTES
+
+SSL_SESSION objects keep internal link information about the session cache
+list, when being inserted into one SSL_CTX object's session cache.
+One SSL_SESSION object, regardless of its reference count, must therefore
+only be used with one SSL_CTX object (and the SSL objects created
+from this SSL_CTX object).
+
 =head1 RETURN VALUES
 
 The following return values can occur:
@@ -42,6 +50,7 @@ The operation succeeded.
 =head1 SEE ALSO
 
 L<ssl(3)|ssl(3)>, L<SSL_SESSION_free(3)|SSL_SESSION_free(3)>,
+L<SSL_get_session(3)|SSL_get_session(3)>,
 L<SSL_session_reused(3)|SSL_session_reused(3)>,
 L<SSL_CTX_set_session_cache_mode(3)|SSL_CTX_set_session_cache_mode(3)>
 

doc/ssl/d2i_SSL_SESSION.pod

@@ -30,7 +30,17 @@ session data on disk or into a database, it must be transformed into
 a binary ASN1 representation.
 
 When using d2i_SSL_SESSION(), the SSL_SESSION object is automatically
-allocated.
+allocated. The reference count is 1, so that the session must be
+explicitly removed using L<SSL_SESSION_free(3)|SSL_SESSION_free(3)>,
+unless the SSL_SESSION object is completely taken over, when being called
+inside the get_session_cb() (see
+L<SSL_CTX_sess_set_get_cb(3)|SSL_CTX_sess_set_get_cb(3)>).
+
+SSL_SESSION objects keep internal link information about the session cache
+list, when being inserted into one SSL_CTX object's session cache.
+One SSL_SESSION object, regardless of its reference count, must therefore
+only be used with one SSL_CTX object (and the SSL objects created
+from this SSL_CTX object).
 
 When using i2d_SSL_SESSION(), the memory location pointed to by B<pp> must be
 large enough to hold the binary representation of the session. There is no
@@ -50,7 +60,7 @@ When the session is not valid, B<0> is returned and no operation is performed.
 
 =head1 SEE ALSO
 
-L<ssl(3)|ssl(3)>,
+L<ssl(3)|ssl(3)>, L<SSL_SESSION_free(3)|SSL_SESSION_free(3)>,
 L<SSL_CTX_sess_set_get_cb(3)|SSL_CTX_sess_set_get_cb(3)>
 
 =cut