#25 null-pointer dereference when using `OTRL_INSTAG_RECENT` with `otrl_message_sending`

[Sven M. Hallberg]

There is a null-pointer dereference bug in libotr 4.0.0 that is triggered by using OTRL_INSTAG_RECENT with otrl_message_sending. When a new OTR context is created via the add_context part in otrl_message_sending, the recent_child field is not set. On the next message otrl_message_sending crashes because even though there is a context, otrl_context_find returns NULL.

The following (untested) patch lets otrl_context_find_recent_instance default to returning the master context when recent_child or one of its friends is not set.

--- a/src/context.c
+++ b/src/context.c
@@ -168,6 +168,7 @@ static ConnContext * new_context(const char * user, const char * accountname,
 ConnContext * otrl_context_find_recent_instance(ConnContext * context,
        otrl_instag_t recent_instag) {
     ConnContext * m_context;
+    ConnContext * recent_child;

     if (!context) return NULL;

@@ -177,14 +178,19 @@ ConnContext * otrl_context_find_recent_instance(ConnContext * context,

     switch(recent_instag) {
        case OTRL_INSTAG_RECENT:
-           return m_context->recent_child;
+           recent_child = m_context->recent_child;
        case OTRL_INSTAG_RECENT_RECEIVED:
-           return m_context->recent_rcvd_child;
+           recent_child = m_context->recent_rcvd_child;
        case OTRL_INSTAG_RECENT_SENT:
-           return m_context->recent_sent_child;
+           recent_child = m_context->recent_sent_child;
        default:
-           return NULL;
+           recent_child = NULL;
     }
+
+    if(recent_child)
+       return recent_child;
+    else
+       return m_context;  /* default to master context */
 }

 /* Find the instance of this context that has the best security level,

After that, one might also consider removing the "dummy" initialization of recent_child et al. when using OTRL_INSTAG_MASTER, though I am not really sure what is intended.

--- a/src/context.c
+++ b/src/context.c
@@ -334,14 +334,6 @@ ConnContext * otrl_context_find(OtrlUserState us, const char *user,
                protocol, OTRL_INSTAG_MASTER, 1, NULL, add_app_data, data);
        }

-       if (their_instance == OTRL_INSTAG_MASTER) {
-           /* if we're adding a master, there are no children, so the most
-            * recent context is the one we add. */
-           newctx->recent_child = newctx;
-           newctx->recent_rcvd_child = newctx;
-           newctx->recent_sent_child = newctx;
-       }
-
        return *curp;
     }
     return NULL;

Cf. also: http://bugs.bitlbee.org/bitlbee/ticket/1110#comment:9