qemu: Avoid using stale data in virDomainGetBlockInfo

CVE-2013-6458

Generally, every API that is going to begin a job should do that before
fetching data from vm->def. However, qemuDomainGetBlockInfo does not
know whether it will have to start a job or not before checking vm->def.
To avoid using disk alias that might have been freed while we were
waiting for a job, we use its copy. In case the disk was removed in the
meantime, we will fail with "cannot find statistics for device '...'"
error message.

(cherry picked from commit b7992595)

Conflicts:
	src/qemu/qemu_driver.c - VIR_STRDUP not backported, context

src/qemu/qemu_driver.c

@@ -9159,6 +9159,7 @@ static int qemuDomainGetBlockInfo(virDomainPtr dom,
     struct stat sb;
     int i;
     int format;
+    char *alias = NULL;
 
     virCheckFlags(0, -1);
 
@@ -9281,13 +9282,18 @@ static int qemuDomainGetBlockInfo(virDomainPtr dom,
         virDomainObjIsActive(vm)) {
         qemuDomainObjPrivatePtr priv = vm->privateData;
 
+        if (!(alias = strdup(disk->info.alias))) {
+            virReportOOMError();
+            goto cleanup;
+        }
+
         if (qemuDomainObjBeginJob(driver, vm, QEMU_JOB_QUERY) < 0)
             goto cleanup;
 
         if (virDomainObjIsActive(vm)) {
             qemuDomainObjEnterMonitor(driver, vm);
             ret = qemuMonitorGetBlockExtent(priv->mon,
-                                            disk->info.alias,
+                                            alias,
                                             &info->allocation);
             qemuDomainObjExitMonitor(driver, vm);
         } else {
@@ -9301,6 +9307,7 @@ static int qemuDomainGetBlockInfo(virDomainPtr dom,
     }
 
 cleanup:
+    VIR_FREE(alias);
     virStorageFileFreeMetadata(meta);
     VIR_FORCE_CLOSE(fd);
     if (vm)