libxl: set XEN_QEMU_CONSOLE_LIMIT for QEMU

XSA-180 provides a patch to QEMU to bodge QEMU logging issue. We
explicitly set the limit in libxl for 4.7.

Introduce a function for setting the environment variable and call it in
the right places.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
(cherry picked from commit b0d409d9c4)

tools/libxl/libxl_dm.c

@@ -363,6 +363,20 @@ int libxl__domain_device_construct_rdm(libxl__gc *gc,
     return ERROR_FAIL;
 }
 
+/* XSA-180 / CVE-2014-3672
+ *
+ * The QEMU shipped with Xen has a bodge. It checks for
+ * XEN_QEMU_CONSOLE_LIMIT to see how much data QEMU is allowed
+ * to write to stderr. We set that to 1MB if it is not set by
+ * system administrator.
+ */
+static void libxl__set_qemu_env_for_xsa_180(libxl__gc *gc,
+                                            flexarray_t *dm_envs)
+{
+    if (getenv("XEN_QEMU_CONSOLE_LIMIT")) return;
+    flexarray_append_pair(dm_envs, "XEN_QEMU_CONSOLE_LIMIT", "1048576");
+}
+
 const libxl_vnc_info *libxl__dm_vnc(const libxl_domain_config *guest_config)
 {
     const libxl_vnc_info *vnc = NULL;
@@ -413,6 +427,8 @@ static int libxl__build_device_model_args_old(libxl__gc *gc,
     dm_args = flexarray_make(gc, 16, 1);
     dm_envs = flexarray_make(gc, 16, 1);
 
+    libxl__set_qemu_env_for_xsa_180(gc, dm_envs);
+
     flexarray_vappend(dm_args, dm,
                       "-d", libxl__sprintf(gc, "%d", domid), NULL);
 
@@ -726,6 +742,8 @@ static int libxl__build_device_model_args_new(libxl__gc *gc,
     dm_args = flexarray_make(gc, 16, 1);
     dm_envs = flexarray_make(gc, 16, 1);
 
+    libxl__set_qemu_env_for_xsa_180(gc, dm_envs);
+
     flexarray_vappend(dm_args, dm,
                       "-xen-domid",
                       libxl__sprintf(gc, "%d", guest_domid), NULL);
@@ -1872,8 +1890,8 @@ static void device_model_spawn_outcome(libxl__egc *egc,
 void libxl__spawn_qdisk_backend(libxl__egc *egc, libxl__dm_spawn_state *dmss)
 {
     STATE_AO_GC(dmss->spawn.ao);
-    flexarray_t *dm_args;
-    char **args;
+    flexarray_t *dm_args, *dm_envs;
+    char **args, **envs;
     const char *dm;
     int logfile_w, null = -1, rc;
     uint32_t domid = dmss->guest_domid;
@@ -1882,6 +1900,8 @@ void libxl__spawn_qdisk_backend(libxl__egc *egc, libxl__dm_spawn_state *dmss)
     dm = qemu_xen_path(gc);
 
     dm_args = flexarray_make(gc, 15, 1);
+    dm_envs = flexarray_make(gc, 1, 1);
+
     flexarray_vappend(dm_args, dm, "-xen-domid",
                       GCSPRINTF("%d", domid), NULL);
     flexarray_append(dm_args, "-xen-attach");
@@ -1895,6 +1915,9 @@ void libxl__spawn_qdisk_backend(libxl__egc *egc, libxl__dm_spawn_state *dmss)
     flexarray_append(dm_args, NULL);
     args = (char **) flexarray_contents(dm_args);
 
+    libxl__set_qemu_env_for_xsa_180(gc, dm_envs);
+    envs = (char **) flexarray_contents(dm_envs);
+
     logfile_w = libxl__create_qemu_logfile(gc, GCSPRINTF("qdisk-%u", domid));
     if (logfile_w < 0) {
         rc = logfile_w;
@@ -1932,7 +1955,7 @@ void libxl__spawn_qdisk_backend(libxl__egc *egc, libxl__dm_spawn_state *dmss)
         goto error;
     if (!rc) { /* inner child */
         setsid();
-        libxl__exec(gc, null, logfile_w, logfile_w, dm, args, NULL);
+        libxl__exec(gc, null, logfile_w, logfile_w, dm, args, envs);
     }
 
     return;