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From 5f881d3bf24599b90d67a45cae7a3eb099cd71c9 Mon Sep 17 00:00:00 2001
From: Bart Van Assche <bvanassche@acm.org>
Date: Sat, 27 Jul 2019 19:34:09 -0700
Subject: [PATCH] libsnmp, USM: Introduce a reference count in struct
 usmStateReference

This patch fixes https://sourceforge.net/p/net-snmp/bugs/2956/.

Upstream-Status: Backport
[ak: fixup for 5.8 context, changes to library/snmpusm.h]
CVE:CVE-2019-20892

Signed-off-by: Armin Kuster <akuster@mvista.com>

---
 snmplib/snmp_client.c | 22 +++----------
 snmplib/snmpusm.c     | 73 ++++++++++++++++++++++++++++---------------
 2 files changed, 53 insertions(+), 42 deletions(-)

Index: net-snmp-5.8/snmplib/snmpusm.c
===================================================================
--- net-snmp-5.8.orig/snmplib/snmpusm.c
+++ net-snmp-5.8/snmplib/snmpusm.c
@@ -285,12 +285,35 @@ free_enginetime_on_shutdown(int majorid,
 struct usmStateReference *
 usm_malloc_usmStateReference(void)
 {
-    struct usmStateReference *retval = (struct usmStateReference *)
-        calloc(1, sizeof(struct usmStateReference));
+   struct usmStateReference *retval;
 
+    retval = calloc(1, sizeof(struct usmStateReference));
+    if (retval)
+        retval->refcnt = 1;
     return retval;
 }                               /* end usm_malloc_usmStateReference() */
 
+static int
+usm_clone(netsnmp_pdu *pdu, netsnmp_pdu *new_pdu)
+{
+    struct usmStateReference *ref = pdu->securityStateRef;
+    struct usmStateReference **new_ref =
+        (struct usmStateReference **)&new_pdu->securityStateRef;
+    int ret = 0;
+
+    if (!ref)
+        return ret;
+
+    if (pdu->command == SNMP_MSG_TRAP2) {
+        netsnmp_assert(pdu->securityModel == SNMP_DEFAULT_SECMODEL);
+        ret = usm_clone_usmStateReference(ref, new_ref);
+    } else {
+        netsnmp_assert(ref == *new_ref);
+        ref->refcnt++;
+    }
+
+    return ret;
+}
 
 void
 usm_free_usmStateReference(void *old)
@@ -3345,6 +3368,7 @@ init_usm(void)
     def->encode_reverse = usm_secmod_rgenerate_out_msg;
     def->encode_forward = usm_secmod_generate_out_msg;
     def->decode = usm_secmod_process_in_msg;
+    def->pdu_clone = usm_clone;
     def->pdu_free_state_ref = usm_free_usmStateReference;
     def->session_setup = usm_session_init;
     def->handle_report = usm_handle_report;
Index: net-snmp-5.8/snmplib/snmp_client.c
===================================================================
--- net-snmp-5.8.orig/snmplib/snmp_client.c
+++ net-snmp-5.8/snmplib/snmp_client.c
@@ -402,27 +402,15 @@ _clone_pdu_header(netsnmp_pdu *pdu)
         return NULL;
     }
 
-    if (pdu->securityStateRef &&
-        pdu->command == SNMP_MSG_TRAP2) {
-
-        ret = usm_clone_usmStateReference((struct usmStateReference *) pdu->securityStateRef,
-                (struct usmStateReference **) &newpdu->securityStateRef );
-
-        if (ret)
-        {
+    sptr = find_sec_mod(newpdu->securityModel);
+    if (sptr && sptr->pdu_clone) {
+        /* call security model if it needs to know about this */
+        ret = sptr->pdu_clone(pdu, newpdu);
+        if (ret) {
             snmp_free_pdu(newpdu);
             return NULL;
         }
     }
-
-    if ((sptr = find_sec_mod(newpdu->securityModel)) != NULL &&
-        sptr->pdu_clone != NULL) {
-        /*
-         * call security model if it needs to know about this 
-         */
-        (*sptr->pdu_clone) (pdu, newpdu);
-    }
-
     return newpdu;
 }
 
Index: net-snmp-5.8/include/net-snmp/library/snmpusm.h
===================================================================
--- net-snmp-5.8.orig/include/net-snmp/library/snmpusm.h
+++ net-snmp-5.8/include/net-snmp/library/snmpusm.h
@@ -43,6 +43,7 @@ extern          "C" {
      * Structures.
      */
     struct usmStateReference {
+        int             refcnt;
         char           *usr_name;
         size_t          usr_name_length;
         u_char         *usr_engine_id;