Memory leak in handle_agentx_packet when using a contextName

[jain-mukesh]

Our infrastructure code uses net-snmp and I found a memory leak in handle_agentx_packet if contextName is given.

Below is the trace from valgrind

==30360==    at 0x4C29EA3: malloc (vg_replace_malloc.c:309)
==30360==    by 0x653F294: snmp_clone_mem (in /usr/lib64/libnetsnmp.so.31.0.2)
==30360==    by 0x653F3E7: ??? (in /usr/lib64/libnetsnmp.so.31.0.2)
==30360==    by 0x653F90D: snmp_clone_pdu (in /usr/lib64/libnetsnmp.so.31.0.2)
==30360==    by 0x60E3837: handle_agentx_packet (in /usr/lib64/libnetsnmpagent.so.31.0.2)
==30360==    by 0x6567ED0: ??? (in /usr/lib64/libnetsnmp.so.31.0.2)
==30360==    by 0x6568F40: _sess_read (in /usr/lib64/libnetsnmp.so.31.0.2)
==30360==    by 0x65698A8: snmp_sess_read2 (in /usr/lib64/libnetsnmp.so.31.0.2)
==30360==    by 0x65698FA: snmp_read2 (in /usr/lib64/libnetsnmp.so.31.0.2)
==30360==    by 0x656993B: snmp_read (in /usr/lib64/libnetsnmp.so.31.0.2)
==30360==    by 0x60DF30A: agent_check_and_process (in /usr/lib64/libnetsnmpagent.so.31.0.2)

On further debugging, we found that the internalPDU->contextName was assigned to the internalPDU->community resulting in losing the reference to the assigned memory of internalPDU->contextName. This memory is never free'ed when the internalPDU is deallocated.

Proposed fix:
contextNames memory should be freed before assignment to community, so that the memory is not lost.

In agent/mibgroup/agentx/subagent.c

internal_pdu = snmp_clone_pdu(pdu);

SNMP_FREE(internal_pdu->contextName);

internal_pdu->contextName = (char *) internal_pdu->community;
internal_pdu->contextNameLen = internal_pdu->community_len;

[bvanassche]

I think the above information shows that the Net-SNMP version on your setup is v5.4, v5.5, v5.6 or v5.7. Please retest with the master branch and report whether or not this leak also occurs with the master branch.

[jain-mukesh]

Thanks @bvanassche . I tested on release 5.8 and 5.3, but will retest this on the master over this week.

[jain-mukesh]

Hi @bvanassche Sorry, it took me a while to integrate the master code with my internal tools. I confirmed that this leak occurs even with the master branch of net-snmp. My proposed fix is in #78

[bvanassche]

Closing this issue since a fix has been applied.