libxl: Restrict permissions on PV console device xenstore nodes

Matthew Daley has observed that the PV console protocol places sensitive host
state into a guest writeable xenstore locations, this includes:

 - The pty used to communicate between the console backend daemon and its
   client, allowing the guest administrator to read and write arbitrary host
   files.
 - The output file, allowing the guest administrator to write arbitrary host
   files or to target arbitrary qemu chardevs which include sockets, udp, ptr,
   pipes etc (see -chardev in qemu(1) for a more complete list).
 - The maximum buffer size, allowing the guest administrator to consume more
   resources than the host administrator has configured.
 - The backend to use (qemu vs xenconsoled), potentially allowing the guest
   administrator to confuse host software.

So we arrange to make the sensitive keys in the xenstore frontend directory
read only for the guest. This is safe since the xenstore permissions model,
unlike POSIX directory permissions, does not allow the guest to remove and
recreate a node if it has write access to the containing directory.

There are a few associated wrinkles:

 - The primary PV console is "special". It's xenstore node is not under the
   usual /devices/ subtree and it does not use the customary xenstore state
   machine protocol. Unfortunately its directory is used for other things,
   including the vnc-port node, which we do not want the guest to be able to
   write to. Rather than trying to track down all the possible secondary uses
   of this directory just make it r/o to the guest. All newly created
   subdirectories inherit these permissions and so are now safe by default.

 - The other serial consoles do use the customary xenstore state machine and
   therefore need write access to at least the "protocol" and "state" nodes,
   however they may also want to use arbitrary "feature-foo" nodes (although
   I'm not aware of any) and therefore we cannot simply lock down the entire
   frontend directory. Instead we add support to libxl__device_generic_add for
   frontend keys which are explicitly read only and use that to lock down the
   sensitive keys.

 - Minios' console frontend wants to write the "type" node, which it has no
   business doing since this is a host/toolstack level decision. This fails
   now that the node has become read only to the PV guest. Since the toolstack
   already writes this node just remove the attempt to set it.

This is a security issue, XSA-57.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>

diff --git a/extras/mini-os/console/xenbus.c b/extras/mini-os/console/xenbus.c
index 77de82a0fa313c7123af51298537385bfb8be08c..e65baf75a970f54fa09378b2fc3b00f507ff905a 100644 (file)
--- a/extras/mini-os/console/xenbus.c
+++ b/extras/mini-os/console/xenbus.c
@@ -122,12 +122,6 @@ again:
         goto abort_transaction;
     }
 
-    err = xenbus_printf(xbt, nodename, "type", "%s", "ioemu");
-    if (err) {
-        message = "writing type";
-        goto abort_transaction;
-    }
-
     snprintf(path, sizeof(path), "%s/state", nodename);
     err = xenbus_switch_state(xbt, path, XenbusStateConnected);
     if (err) {

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index ee1fa9cf8c91cc7c462c5679fc3bc3cdb50554d4..0612d85a24755422d1360c3a30d2449f657edf2f 100644 (file)
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -1813,8 +1813,9 @@ void libxl__device_vtpm_add(libxl__egc *egc, uint32_t domid,
     flexarray_append(front, GCSPRINTF("%d", vtpm->devid));
 
     libxl__device_generic_add(gc, XBT_NULL, device,
-                             libxl__xs_kvs_of_flexarray(gc, back, back->count),
-                             libxl__xs_kvs_of_flexarray(gc, front, front->count));
+                              libxl__xs_kvs_of_flexarray(gc, back, back->count),
+                              libxl__xs_kvs_of_flexarray(gc, front, front->count),
+                              NULL);
 
     aodev->dev = device;
     aodev->action = LIBXL__DEVICE_ACTION_ADD;
@@ -2195,8 +2196,9 @@ static void device_disk_add(libxl__egc *egc, uint32_t domid,
         }
 
         libxl__device_generic_add(gc, t, device,
-                            libxl__xs_kvs_of_flexarray(gc, back, back->count),
-                            libxl__xs_kvs_of_flexarray(gc, front, front->count));
+                                  libxl__xs_kvs_of_flexarray(gc, back, back->count),
+                                  libxl__xs_kvs_of_flexarray(gc, front, front->count),
+                                  NULL);
 
         rc = libxl__xs_transaction_commit(gc, &t);
         if (!rc) break;
@@ -2938,8 +2940,9 @@ void libxl__device_nic_add(libxl__egc *egc, uint32_t domid,
     flexarray_append(front, libxl__sprintf(gc,
                                     LIBXL_MAC_FMT, LIBXL_MAC_BYTES(nic->mac)));
     libxl__device_generic_add(gc, XBT_NULL, device,
-                             libxl__xs_kvs_of_flexarray(gc, back, back->count),
-                             libxl__xs_kvs_of_flexarray(gc, front, front->count));
+                              libxl__xs_kvs_of_flexarray(gc, back, back->count),
+                              libxl__xs_kvs_of_flexarray(gc, front, front->count),
+                              NULL);
 
     aodev->dev = device;
     aodev->action = LIBXL__DEVICE_ACTION_ADD;
@@ -3132,7 +3135,7 @@ int libxl__device_console_add(libxl__gc *gc, uint32_t domid,
                               libxl__device_console *console,
                               libxl__domain_build_state *state)
 {
-    flexarray_t *front;
+    flexarray_t *front, *ro_front;
     flexarray_t *back;
     libxl__device device;
     int rc;
@@ -3143,6 +3146,7 @@ int libxl__device_console_add(libxl__gc *gc, uint32_t domid,
     }
 
     front = flexarray_make(gc, 16, 1);
+    ro_front = flexarray_make(gc, 16, 1);
     back = flexarray_make(gc, 16, 1);
 
     device.backend_devid = console->devid;
@@ -3165,21 +3169,24 @@ int libxl__device_console_add(libxl__gc *gc, uint32_t domid,
 
     flexarray_append(front, "backend-id");
     flexarray_append(front, libxl__sprintf(gc, "%d", console->backend_domid));
-    flexarray_append(front, "limit");
-    flexarray_append(front, libxl__sprintf(gc, "%d", LIBXL_XENCONSOLE_LIMIT));
-    flexarray_append(front, "type");
+
+    flexarray_append(ro_front, "limit");
+    flexarray_append(ro_front, libxl__sprintf(gc, "%d", LIBXL_XENCONSOLE_LIMIT));
+    flexarray_append(ro_front, "type");
     if (console->consback == LIBXL__CONSOLE_BACKEND_XENCONSOLED)
-        flexarray_append(front, "xenconsoled");
+        flexarray_append(ro_front, "xenconsoled");
     else
-        flexarray_append(front, "ioemu");
-    flexarray_append(front, "output");
-    flexarray_append(front, console->output);
+        flexarray_append(ro_front, "ioemu");
+    flexarray_append(ro_front, "output");
+    flexarray_append(ro_front, console->output);
+    flexarray_append(ro_front, "tty");
+    flexarray_append(ro_front, "");
 
     if (state) {
-        flexarray_append(front, "port");
-        flexarray_append(front, libxl__sprintf(gc, "%"PRIu32, state->console_port));
-        flexarray_append(front, "ring-ref");
-        flexarray_append(front, libxl__sprintf(gc, "%lu", state->console_mfn));
+        flexarray_append(ro_front, "port");
+        flexarray_append(ro_front, libxl__sprintf(gc, "%"PRIu32, state->console_port));
+        flexarray_append(ro_front, "ring-ref");
+        flexarray_append(ro_front, libxl__sprintf(gc, "%lu", state->console_mfn));
     } else {
         flexarray_append(front, "state");
         flexarray_append(front, libxl__sprintf(gc, "%d", 1));
@@ -3188,8 +3195,9 @@ int libxl__device_console_add(libxl__gc *gc, uint32_t domid,
     }
 
     libxl__device_generic_add(gc, XBT_NULL, &device,
-                             libxl__xs_kvs_of_flexarray(gc, back, back->count),
-                             libxl__xs_kvs_of_flexarray(gc, front, front->count));
+                              libxl__xs_kvs_of_flexarray(gc, back, back->count),
+                              libxl__xs_kvs_of_flexarray(gc, front, front->count),
+                              libxl__xs_kvs_of_flexarray(gc, ro_front, ro_front->count));
     rc = 0;
 out:
     return rc;
@@ -3274,8 +3282,9 @@ int libxl__device_vkb_add(libxl__gc *gc, uint32_t domid,
     flexarray_append(front, libxl__sprintf(gc, "%d", 1));
 
     libxl__device_generic_add(gc, XBT_NULL, &device,
-                             libxl__xs_kvs_of_flexarray(gc, back, back->count),
-                             libxl__xs_kvs_of_flexarray(gc, front, front->count));
+                              libxl__xs_kvs_of_flexarray(gc, back, back->count),
+                              libxl__xs_kvs_of_flexarray(gc, front, front->count),
+                              NULL);
     rc = 0;
 out:
     return rc;
@@ -3387,8 +3396,9 @@ int libxl__device_vfb_add(libxl__gc *gc, uint32_t domid, libxl_device_vfb *vfb)
     flexarray_append_pair(front, "state", libxl__sprintf(gc, "%d", 1));
 
     libxl__device_generic_add(gc, XBT_NULL, &device,
-                             libxl__xs_kvs_of_flexarray(gc, back, back->count),
-                             libxl__xs_kvs_of_flexarray(gc, front, front->count));
+                              libxl__xs_kvs_of_flexarray(gc, back, back->count),
+                              libxl__xs_kvs_of_flexarray(gc, front, front->count),
+                              NULL);
     rc = 0;
 out:
     return rc;

diff --git a/tools/libxl/libxl_device.c b/tools/libxl/libxl_device.c
index bc86648af80244d5144d508539bb243747370d6a..ea845b733475b0e7de44178b54becaf04447c81c 100644 (file)
--- a/tools/libxl/libxl_device.c
+++ b/tools/libxl/libxl_device.c
@@ -84,11 +84,12 @@ out:
 }
 
 int libxl__device_generic_add(libxl__gc *gc, xs_transaction_t t,
-        libxl__device *device, char **bents, char **fents)
+        libxl__device *device, char **bents, char **fents, char **ro_fents)
 {
     libxl_ctx *ctx = libxl__gc_owner(gc);
     char *frontend_path, *backend_path;
     struct xs_permissions frontend_perms[2];
+    struct xs_permissions ro_frontend_perms[2];
     struct xs_permissions backend_perms[2];
     int create_transaction = t == XBT_NULL;
 
@@ -100,22 +101,37 @@ int libxl__device_generic_add(libxl__gc *gc, xs_transaction_t t,
     frontend_perms[1].id = device->backend_domid;
     frontend_perms[1].perms = XS_PERM_READ;
 
-    backend_perms[0].id = device->backend_domid;
-    backend_perms[0].perms = XS_PERM_NONE;
-    backend_perms[1].id = device->domid;
-    backend_perms[1].perms = XS_PERM_READ;
+    ro_frontend_perms[0].id = backend_perms[0].id = device->backend_domid;
+    ro_frontend_perms[0].perms = backend_perms[0].perms = XS_PERM_NONE;
+    ro_frontend_perms[1].id = backend_perms[1].id = device->domid;
+    ro_frontend_perms[1].perms = backend_perms[1].perms = XS_PERM_READ;
 
 retry_transaction:
     if (create_transaction)
         t = xs_transaction_start(ctx->xsh);
     /* FIXME: read frontend_path and check state before removing stuff */
 
-    if (fents) {
+    if (fents || ro_fents) {
         xs_rm(ctx->xsh, t, frontend_path);
         xs_mkdir(ctx->xsh, t, frontend_path);
-        xs_set_permissions(ctx->xsh, t, frontend_path, frontend_perms, ARRAY_SIZE(frontend_perms));
+        /* Console 0 is a special case. It doesn't use the regular PV
+         * state machine but also the frontend directory has
+         * historically contained other information, such as the
+         * vnc-port, which we don't want the guest fiddling with.
+         */
+        if (device->kind == LIBXL__DEVICE_KIND_CONSOLE && device->devid == 0)
+            xs_set_permissions(ctx->xsh, t, frontend_path,
+                               ro_frontend_perms, ARRAY_SIZE(ro_frontend_perms));
+        else
+            xs_set_permissions(ctx->xsh, t, frontend_path,
+                               frontend_perms, ARRAY_SIZE(frontend_perms));
         xs_write(ctx->xsh, t, libxl__sprintf(gc, "%s/backend", frontend_path), backend_path, strlen(backend_path));
-        libxl__xs_writev(gc, t, frontend_path, fents);
+        if (fents)
+            libxl__xs_writev_perms(gc, t, frontend_path, fents,
+                                   frontend_perms, ARRAY_SIZE(frontend_perms));
+        if (ro_fents)
+            libxl__xs_writev_perms(gc, t, frontend_path, ro_fents,
+                                   ro_frontend_perms, ARRAY_SIZE(ro_frontend_perms));
     }
 
     if (bents) {

diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 3ba3a21850ccac9c6d9dc7ef67ed66b8d67ce42c..00ff6b96cdbcf61acaceb7496e2c274357aef4ab 100644 (file)
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -519,6 +519,11 @@ _hidden char **libxl__xs_kvs_of_flexarray(libxl__gc *gc, flexarray_t *array, int
 /* treats kvs as pairs of keys and values and writes each to dir. */
 _hidden int libxl__xs_writev(libxl__gc *gc, xs_transaction_t t,
                              const char *dir, char **kvs);
+/* as writev but also sets the permissions on each path */
+_hidden int libxl__xs_writev_perms(libxl__gc *gc, xs_transaction_t t,
+                                   const char *dir, char *kvs[],
+                                   struct xs_permissions *perms,
+                                   unsigned int num_perms);
 /* _atonce creates a transaction and writes all keys at once */
 _hidden int libxl__xs_writev_atonce(libxl__gc *gc,
                              const char *dir, char **kvs);
@@ -933,7 +938,7 @@ _hidden int libxl__device_console_add(libxl__gc *gc, uint32_t domid,
                                       libxl__domain_build_state *state);
 
 _hidden int libxl__device_generic_add(libxl__gc *gc, xs_transaction_t t,
-        libxl__device *device, char **bents, char **fents);
+        libxl__device *device, char **bents, char **fents, char **ro_fents);
 _hidden char *libxl__device_backend_path(libxl__gc *gc, libxl__device *device);
 _hidden char *libxl__device_frontend_path(libxl__gc *gc, libxl__device *device);
 _hidden int libxl__parse_backend_path(libxl__gc *gc, const char *path,

diff --git a/tools/libxl/libxl_pci.c b/tools/libxl/libxl_pci.c
index eac35c10f204062499511a3b670aac81ee483cb9..2f9f010cb3b30480ecdae6f8f69615b18035173a 100644 (file)
--- a/tools/libxl/libxl_pci.c
+++ b/tools/libxl/libxl_pci.c
@@ -102,7 +102,8 @@ int libxl__create_pci_backend(libxl__gc *gc, uint32_t domid,
 
     libxl__device_generic_add(gc, XBT_NULL, &device,
                               libxl__xs_kvs_of_flexarray(gc, back, back->count),
-                              libxl__xs_kvs_of_flexarray(gc, front, front->count));
+                              libxl__xs_kvs_of_flexarray(gc, front, front->count),
+                              NULL);
 
     return 0;
 }

diff --git a/tools/libxl/libxl_xshelp.c b/tools/libxl/libxl_xshelp.c
index 52af4842e4a1675c6ab76f9269ec1976941264e5..d7eaa660bbe1e62da26f33e283ef1e05906b68db 100644 (file)
--- a/tools/libxl/libxl_xshelp.c
+++ b/tools/libxl/libxl_xshelp.c
@@ -41,8 +41,10 @@ char **libxl__xs_kvs_of_flexarray(libxl__gc *gc, flexarray_t *array, int length)
     return kvs;
 }
 
-int libxl__xs_writev(libxl__gc *gc, xs_transaction_t t,
-                     const char *dir, char *kvs[])
+int libxl__xs_writev_perms(libxl__gc *gc, xs_transaction_t t,
+                           const char *dir, char *kvs[],
+                           struct xs_permissions *perms,
+                           unsigned int num_perms)
 {
     libxl_ctx *ctx = libxl__gc_owner(gc);
     char *path;
@@ -56,11 +58,19 @@ int libxl__xs_writev(libxl__gc *gc, xs_transaction_t t,
         if (path && kvs[i + 1]) {
             int length = strlen(kvs[i + 1]);
             xs_write(ctx->xsh, t, path, kvs[i + 1], length);
+            if (perms)
+                xs_set_permissions(ctx->xsh, t, path, perms, num_perms);
         }
     }
     return 0;
 }
 
+int libxl__xs_writev(libxl__gc *gc, xs_transaction_t t,
+                     const char *dir, char *kvs[])
+{
+    return libxl__xs_writev_perms(gc, t, dir, kvs, NULL, 0);
+}
+
 int libxl__xs_writev_atonce(libxl__gc *gc,
                             const char *dir, char *kvs[])
 {