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From 4589352dac3ae111c7621298cf231742209efd9b Mon Sep 17 00:00:00 2001
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From: Bill Fenner <fenner@gmail.com>
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Date: Fri, 25 Nov 2022 08:41:24 -0800
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Subject: [PATCH 1/3] snmp_agent: disallow SET with NULL varbind
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---
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 agent/snmp_agent.c | 32 ++++++++++++++++++++++++++++++++
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 1 file changed, 32 insertions(+)
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diff --git a/agent/snmp_agent.c b/agent/snmp_agent.c
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index 867d0c166f..3f678fe2df 100644
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--- a/agent/snmp_agent.c
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+++ b/agent/snmp_agent.c
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@@ -3719,12 +3719,44 @@ netsnmp_handle_request(netsnmp_agent_session *asp, int status)
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     return 1;
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 }
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+static int
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+check_set_pdu_for_null_varbind(netsnmp_agent_session *asp)
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+{
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+    int i;
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+    netsnmp_variable_list *v = NULL;
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+
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+    for (i = 1, v = asp->pdu->variables; v != NULL; i++, v = v->next_variable) {
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+	if (v->type == ASN_NULL) {
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+	    /*
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+	     * Protect SET implementations that do not protect themselves
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+	     * against wrong type.
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+	     */
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+	    DEBUGMSGTL(("snmp_agent", "disallowing SET with NULL var for varbind %d\n", i));
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+	    asp->index = i;
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+	    return SNMP_ERR_WRONGTYPE;
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+	}
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+    }
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+    return SNMP_ERR_NOERROR;
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+}
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+
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 int
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 handle_pdu(netsnmp_agent_session *asp)
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 {
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     int             status, inclusives = 0;
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     netsnmp_variable_list *v = NULL;
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+#ifndef NETSNMP_NO_WRITE_SUPPORT
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+    /*
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+     * Check for ASN_NULL in SET request
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+     */
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+    if (asp->pdu->command == SNMP_MSG_SET) {
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+	status = check_set_pdu_for_null_varbind(asp);
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+	if (status != SNMP_ERR_NOERROR) {
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+	    return status;
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+	}
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+    }
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+#endif /* NETSNMP_NO_WRITE_SUPPORT */
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+
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     /*
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      * for illegal requests, mark all nodes as ASN_NULL 
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      */
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From 7f4ac4051cc7fec6a5944661923acb95cec359c7 Mon Sep 17 00:00:00 2001
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From: Bill Fenner <fenner@gmail.com>
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Date: Fri, 25 Nov 2022 08:41:46 -0800
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Subject: [PATCH 2/3] apps: snmpset: allow SET with NULL varbind for testing
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---
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 apps/snmpset.c | 1 +
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 1 file changed, 1 insertion(+)
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diff --git a/apps/snmpset.c b/apps/snmpset.c
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index 48e14bd513..d542713e1b 100644
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--- a/apps/snmpset.c
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+++ b/apps/snmpset.c
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@@ -182,6 +182,7 @@ main(int argc, char *argv[])
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             case 'x':
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             case 'd':
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             case 'b':
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+            case 'n': /* undocumented */
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 #ifdef NETSNMP_WITH_OPAQUE_SPECIAL_TYPES
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             case 'I':
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             case 'U':
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From 15f9d7f7e5b90c9b419832ed8e6413feb6570d83 Mon Sep 17 00:00:00 2001
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From: Bill Fenner <fenner@gmail.com>
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Date: Fri, 25 Nov 2022 10:23:32 -0800
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Subject: [PATCH 3/3] Add test for NULL varbind set
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---
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 .../default/T0142snmpv2csetnull_simple        | 31 +++++++++++++++++++
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 1 file changed, 31 insertions(+)
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 create mode 100644 testing/fulltests/default/T0142snmpv2csetnull_simple
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diff --git a/testing/fulltests/default/T0142snmpv2csetnull_simple b/testing/fulltests/default/T0142snmpv2csetnull_simple
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new file mode 100644
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index 0000000000..0f1b8f386b
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--- /dev/null
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+++ b/testing/fulltests/default/T0142snmpv2csetnull_simple
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@@ -0,0 +1,31 @@
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+#!/bin/sh
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+
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+. ../support/simple_eval_tools.sh
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+
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+HEADER SNMPv2c set of system.sysContact.0 with NULL varbind
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+
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+SKIPIF NETSNMP_DISABLE_SET_SUPPORT
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+SKIPIF NETSNMP_NO_WRITE_SUPPORT
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+SKIPIF NETSNMP_DISABLE_SNMPV2C
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+SKIPIFNOT USING_MIBII_SYSTEM_MIB_MODULE
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+
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+#
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+# Begin test
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+#
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+
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+# standard V2C configuration: testcomunnity
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+snmp_write_access='all'
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+. ./Sv2cconfig
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+STARTAGENT
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+
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+CAPTURE "snmpget -On $SNMP_FLAGS -c testcommunity -v 2c $SNMP_TRANSPORT_SPEC:$SNMP_TEST_DEST$SNMP_SNMPD_PORT .1.3.6.1.2.1.1.4.0"
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+
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+CHECK ".1.3.6.1.2.1.1.4.0 = STRING:"
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+
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+CAPTURE "snmpset -On $SNMP_FLAGS -c testcommunity -v 2c $SNMP_TRANSPORT_SPEC:$SNMP_TEST_DEST$SNMP_SNMPD_PORT .1.3.6.1.2.1.1.4.0 n x"
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+
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+CHECK "Reason: wrongType"
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+
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+STOPAGENT
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+
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+FINISHED
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