qemu: Prevent crash of libvirtd without guest agent configuration

CVE-2013-4154

If users haven't configured guest agent then qemuAgentCommand() will
dereference a NULL 'mon' pointer, which causes crash of libvirtd when
using agent based cpu (un)plug.

With the patch, when the qemu-ga service isn't running in the guest,
a expected error "error: Guest agent is not responding: Guest agent
not available for now" will be raised, and the error "error: argument
unsupported: QEMU guest agent is not configured" is raised when the
guest hasn't configured guest agent.

GDB backtrace:

 (gdb) bt
 #0  virNetServerFatalSignal (sig=11, siginfo=<value optimized out>, context=<value optimized out>) at rpc/virnetserver.c:326
 #1  <signal handler called>
 #2  qemuAgentCommand (mon=0x0, cmd=0x7f39300017b0, reply=0x7f394b090910, seconds=-2) at qemu/qemu_agent.c:975
 #3  0x00007f39429507f6 in qemuAgentGetVCPUs (mon=0x0, info=0x7f394b0909b8) at qemu/qemu_agent.c:1475
 #4  0x00007f39429d9857 in qemuDomainGetVcpusFlags (dom=<value optimized out>, flags=9) at qemu/qemu_driver.c:4849
 #5  0x00007f3957dffd8d in virDomainGetVcpusFlags (domain=0x7f39300009c0, flags=8) at libvirt.c:9843

How to reproduce?

 # To start a guest without guest agent configuration
 # then run the following cmdline

 # virsh vcpucount foobar --guest
 error: End of file while reading data: Input/output error
 error: One or more references were leaked after disconnect from the hypervisor
 error: Failed to reconnect to the hypervisor

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=984821Signed-off-by: NAlex Jia <ajia@redhat.com>
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
(cherry picked from commit 96518d43)

src/qemu/qemu_driver.c

@@ -3963,6 +3963,19 @@ qemuDomainSetVcpusFlags(virDomainPtr dom, unsigned int nvcpus,
             goto endjob;
         }
 
+        if (priv->agentError) {
+            virReportError(VIR_ERR_AGENT_UNRESPONSIVE, "%s",
+                           _("QEMU guest agent is not "
+                             "available due to an error"));
+            goto endjob;
+        }
+
+        if (!priv->agent) {
+            virReportError(VIR_ERR_ARGUMENT_UNSUPPORTED, "%s",
+                           _("QEMU guest agent is not configured"));
+            goto endjob;
+        }
+
         qemuDomainObjEnterAgent(vm);
         ncpuinfo = qemuAgentGetVCPUs(priv->agent, &cpuinfo);
         qemuDomainObjExitAgent(vm);
@@ -4685,6 +4698,19 @@ qemuDomainGetVcpusFlags(virDomainPtr dom, unsigned int flags)
         if (qemuDomainObjBeginJob(driver, vm, QEMU_JOB_QUERY) < 0)
             goto cleanup;
 
+        if (priv->agentError) {
+            virReportError(VIR_ERR_AGENT_UNRESPONSIVE, "%s",
+                           _("QEMU guest agent is not "
+                             "available due to an error"));
+            goto endjob;
+        }
+
+        if (!priv->agent) {
+            virReportError(VIR_ERR_ARGUMENT_UNSUPPORTED, "%s",
+                           _("QEMU guest agent is not configured"));
+            goto endjob;
+        }
+
         if (!virDomainObjIsActive(vm)) {
             virReportError(VIR_ERR_OPERATION_INVALID, "%s",
                            _("domain is not running"));