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 CVE-2013-2211 / XSA-57

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

Conflicts (4.2 backport):
	tools/libxl/libxl.c (no vtpm, free front_ro on error in
	                     libxl__device_console_add)

Conflicts (4.1 backport):
	extras/mini-os/console/xenbus.c
	tools/libxl/libxl.c
	tools/libxl/libxl_device.c
	tools/libxl/libxl_internal.h
	tools/libxl/libxl_pci.c
	tools/libxl/libxl_xshelp.c

  - minios code was in xencons_ring.c
  - many places need &gc not just gc
  - libxl__xs_writev path is not const
  - varios minor context fixups

6 files changed, 83 insertions, 43 deletions

diff --git a/extras/mini-os/console/xencons_ring.c b/extras/mini-os/console/xencons_ring.c
index 9ed37567fd..286c650a25 100644
--- a/extras/mini-os/console/xencons_ring.c
+++ b/extras/mini-os/console/xencons_ring.c
@@ -291,12 +291,6 @@ 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 3c2e1b2175..54f440c749 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -1036,8 +1036,9 @@ int libxl_device_disk_add(libxl_ctx *ctx, uint32_t domid, libxl_device_disk *dis
     }
 
     libxl__device_generic_add(ctx, &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;
 
@@ -1266,8 +1267,9 @@ int libxl_device_nic_add(libxl_ctx *ctx, uint32_t domid, libxl_device_nic *nic)
     }
 
     libxl__device_generic_add(ctx, &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);
 
     /* FIXME: wait for plug */
     rc = 0;
@@ -1478,8 +1480,9 @@ int libxl_device_net2_add(libxl_ctx *ctx, uint32_t domid, libxl_device_net2 *net
     flexarray_append(front, "1");
 
     libxl__device_generic_add(ctx, &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);
 
     /* FIXME: wait for plug */
     rc = 0;
@@ -1571,7 +1574,7 @@ int libxl_device_net2_del(libxl_ctx *ctx, libxl_device_net2 *net2, int wait)
 int libxl_device_console_add(libxl_ctx *ctx, uint32_t domid, libxl_device_console *console)
 {
     libxl__gc gc = LIBXL_INIT_GC(ctx);
-    flexarray_t *front;
+    flexarray_t *front, *ro_front;
     flexarray_t *back;
     libxl__device device;
     int rc;
@@ -1581,6 +1584,11 @@ int libxl_device_console_add(libxl_ctx *ctx, uint32_t domid, libxl_device_consol
         rc = ERROR_NOMEM;
         goto out;
     }
+    ro_front = flexarray_make(16, 1);
+    if (!ro_front) {
+        rc = ERROR_NOMEM;
+        goto out;
+    }
     back = flexarray_make(16, 1);
     if (!back) {
         rc = ERROR_NOMEM;
@@ -1607,25 +1615,27 @@ int libxl_device_console_add(libxl_ctx *ctx, uint32_t domid, libxl_device_consol
 
     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_CONSBACK_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 (device.devid == 0) {
         if (console->build_state == NULL) {
             rc = ERROR_INVAL;
             goto out_free;
         }
-        flexarray_append(front, "port");
-        flexarray_append(front, libxl__sprintf(&gc, "%"PRIu32, console->build_state->console_port));
-        flexarray_append(front, "ring-ref");
-        flexarray_append(front, libxl__sprintf(&gc, "%lu", console->build_state->console_mfn));
+        flexarray_append(ro_front, "port");
+        flexarray_append(ro_front, libxl__sprintf(&gc, "%"PRIu32, console->build_state->console_port));
+        flexarray_append(ro_front, "ring-ref");
+        flexarray_append(ro_front, libxl__sprintf(&gc, "%lu", console->build_state->console_mfn));
     } else {
         flexarray_append(front, "state");
         flexarray_append(front, libxl__sprintf(&gc, "%d", 1));
@@ -1634,11 +1644,13 @@ int libxl_device_console_add(libxl_ctx *ctx, uint32_t domid, libxl_device_consol
     }
 
     libxl__device_generic_add(ctx, &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_free:
     flexarray_free(back);
+    flexarray_free(ro_front);
     flexarray_free(front);
 out:
     libxl__free_all(&gc);
@@ -1693,8 +1705,9 @@ int libxl_device_vkb_add(libxl_ctx *ctx, uint32_t domid, libxl_device_vkb *vkb)
     flexarray_append(front, libxl__sprintf(&gc, "%d", 1));
 
     libxl__device_generic_add(ctx, &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_free:
     flexarray_free(back);
@@ -1921,8 +1934,9 @@ int libxl_device_vfb_add(libxl_ctx *ctx, uint32_t domid, libxl_device_vfb *vfb)
     flexarray_append_pair(front, "state", libxl__sprintf(&gc, "%d", 1));
 
     libxl__device_generic_add(ctx, &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_free:
     flexarray_free(front);
diff --git a/tools/libxl/libxl_device.c b/tools/libxl/libxl_device.c
index 7e8fcef20d..06288405f9 100644
--- a/tools/libxl/libxl_device.c
+++ b/tools/libxl/libxl_device.c
@@ -62,12 +62,13 @@ char *libxl__device_backend_path(libxl__gc *gc, libxl__device *device)
 }
 
 int libxl__device_generic_add(libxl_ctx *ctx, libxl__device *device,
-                             char **bents, char **fents)
+                              char **bents, char **fents, char **ro_fents)
 {
     libxl__gc gc = LIBXL_INIT_GC(ctx);
     char *frontend_path, *backend_path;
     xs_transaction_t t;
     struct xs_permissions frontend_perms[2];
+    struct xs_permissions ro_frontend_perms[2];
     struct xs_permissions backend_perms[2];
     int rc;
 
@@ -84,21 +85,36 @@ int libxl__device_generic_add(libxl_ctx *ctx, libxl__device *device,
     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:
     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 == DEVICE_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 9cf503f20e..5ddd27b02d 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -143,6 +143,11 @@ _hidden char **libxl__xs_kvs_of_flexarray(libxl__gc *gc, flexarray_t *array, int
 
 _hidden int libxl__xs_writev(libxl__gc *gc, xs_transaction_t t,
                     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,
+                                   char *dir, char *kvs[],
+                                   struct xs_permissions *perms,
+                                   unsigned int num_perms);
 _hidden int libxl__xs_write(libxl__gc *gc, xs_transaction_t t,
                    char *path, char *fmt, ...) PRINTF_ATTRIBUTE(4, 5);
    /* Each fn returns 0 on success.
@@ -185,7 +190,7 @@ _hidden int libxl__device_physdisk_major_minor(const char *physpath, int *major,
 _hidden int libxl__device_disk_dev_number(const char *virtpath);
 
 _hidden int libxl__device_generic_add(libxl_ctx *ctx, libxl__device *device,
-                             char **bents, char **fents);
+                                      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__device_del(libxl_ctx *ctx, libxl__device *dev, int wait);
diff --git a/tools/libxl/libxl_pci.c b/tools/libxl/libxl_pci.c
index b1d05d9e39..9c76bceaa2 100644
--- a/tools/libxl/libxl_pci.c
+++ b/tools/libxl/libxl_pci.c
@@ -274,8 +274,9 @@ static int libxl_create_pci_backend(libxl__gc *gc, uint32_t domid, libxl_device_
     flexarray_append_pair(front, "state", libxl__sprintf(gc, "%d", 1));
 
     libxl__device_generic_add(ctx, &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);
 
 out:
     if (back)
diff --git a/tools/libxl/libxl_xshelp.c b/tools/libxl/libxl_xshelp.c
index 3dc9239bcb..06b95e0851 100644
--- a/tools/libxl/libxl_xshelp.c
+++ b/tools/libxl/libxl_xshelp.c
@@ -48,8 +48,10 @@ 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,
-                    char *dir, char *kvs[])
+int libxl__xs_writev_perms(libxl__gc *gc, xs_transaction_t t,
+                           char *dir, char *kvs[],
+                           struct xs_permissions *perms,
+                           unsigned int num_perms)
 {
     libxl_ctx *ctx = libxl__gc_owner(gc);
     char *path;
@@ -63,11 +65,19 @@ 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,
+                    char *dir, char *kvs[])
+{
+    return libxl__xs_writev_perms(gc, t, dir, kvs, NULL, 0);
+}
+
 int libxl__xs_write(libxl__gc *gc, xs_transaction_t t,
                    char *path, char *fmt, ...)
 {