QEMU running under Xen doesn't need mount or IPC functionality.
Create and enter separate namespaces for each of these before
executing QEMU, so that in the event that other restrictions fail, the
process won't be able to even name system mount points or exsting
non-file-based IPC descriptors to attempt to attack them.

Unsharing is something a process can only do to itself (it would
seem); so add an os-specific "dm_preexec_restrict()" hook just before
we exec() the device model.

Also add checks to depriv-process-checker.sh to verify that dm is
running in a new namespace (or at least, a different one than the
caller).

Suggested-by: Ross Lagerwall <ross.lagerw...@citrix.com>
Signed-off-by: George Dunlap <george.dun...@citrix.com>
Acked-by: Ian Jackson <ian.jack...@eu.citrix.com>
---
Changes since v3:
- Fix some more style issues

Changes since v2:
- Return an error rather than calling exit()
- Use LOGE() and print to the current stderr fd, rather than
  printing to the new stderr fd via write()
- Use r for external return values rather than rc.

CC: Ian Jackson <ian.jack...@citrix.com>
CC: Wei Liu <wei.l...@citrix.com>
CC: Anthony Perard <anthony.per...@citrix.com>
---
 docs/designs/qemu-deprivilege.md | 12 ++++++------
 tools/libxl/libxl_dm.c           |  5 +++++
 tools/libxl/libxl_freebsd.c      |  5 +++++
 tools/libxl/libxl_internal.h     |  5 +++++
 tools/libxl/libxl_linux.c        | 14 ++++++++++++++
 tools/libxl/libxl_netbsd.c       |  5 +++++
 6 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/docs/designs/qemu-deprivilege.md b/docs/designs/qemu-deprivilege.md
index 0395bbbb40..a461ebbadd 100644
--- a/docs/designs/qemu-deprivilege.md
+++ b/docs/designs/qemu-deprivilege.md
@@ -78,12 +78,6 @@ Then adds the following to the qemu command-line:
        
 '''Tested''': Not tested
 
-## Restrictions / improvements still to do
-
-This lists potential restrictions still to do.  It is meant to be
-listed in order of ease of implementation, with low-hanging fruit
-first.
-
 ## Namespaces for unused functionality (Linux only)
 
 '''Description''': QEMU doesn't use the functionality associated with
@@ -111,6 +105,12 @@ call:
 
 [qemu-namespaces]: 
https://lists.gnu.org/archive/html/qemu-devel/2017-10/msg04723.html
 
+# Restrictions / improvements still to do
+
+This lists potential restrictions still to do.  It is meant to be
+listed in order of ease of implementation, with low-hanging fruit
+first.
+
 ### Basic RLIMITs
 
 '''Description''': A number of limits on the resources that a given
diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index ad3efcc783..278cfd6e6e 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -2393,6 +2393,11 @@ retry_transaction:
         goto out_close;
     if (!rc) { /* inner child */
         setsid();
+        if (libxl_defbool_val(b_info->dm_restrict)) {
+            rc = libxl__local_dm_preexec_restrict(gc);
+            if (rc)
+                _exit(-1);
+        }
         libxl__exec(gc, null, logfile_w, logfile_w, dm, args, envs);
     }
 
diff --git a/tools/libxl/libxl_freebsd.c b/tools/libxl/libxl_freebsd.c
index 6442ccec72..f7ef4a8910 100644
--- a/tools/libxl/libxl_freebsd.c
+++ b/tools/libxl/libxl_freebsd.c
@@ -245,3 +245,8 @@ int libxl__pci_topology_init(libxl__gc *gc,
 {
     return ERROR_NI;
 }
+
+int libxl__local_dm_preexec_restrict(libxl__gc *gc)
+{
+    return 0;
+}
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index ff889385fe..e498435e16 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -3774,6 +3774,11 @@ struct libxl__dm_spawn_state {
 
 _hidden void libxl__spawn_local_dm(libxl__egc *egc, libxl__dm_spawn_state*);
 
+/* 
+ * Called after forking but before executing the local devicemodel.
+ */
+_hidden int libxl__local_dm_preexec_restrict(libxl__gc *gc);
+
 /* Stubdom device models. */
 
 typedef struct {
diff --git a/tools/libxl/libxl_linux.c b/tools/libxl/libxl_linux.c
index 6ef0abc693..c7a345f4bb 100644
--- a/tools/libxl/libxl_linux.c
+++ b/tools/libxl/libxl_linux.c
@@ -307,6 +307,20 @@ int libxl__pci_topology_init(libxl__gc *gc,
     return err;
 }
 
+int libxl__local_dm_preexec_restrict(libxl__gc *gc)
+{
+    int r;
+
+    /* Unshare mount and IPC namespaces.  These are unused by QEMU. */
+    r = unshare(CLONE_NEWNS | CLONE_NEWIPC);
+    if (r) {
+        LOGE(ERROR, "libxl: Mount and IPC namespace unfailed");
+        return ERROR_FAIL;
+    }
+
+    return 0;
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/tools/libxl/libxl_netbsd.c b/tools/libxl/libxl_netbsd.c
index 2edfb00641..dce3f1fdce 100644
--- a/tools/libxl/libxl_netbsd.c
+++ b/tools/libxl/libxl_netbsd.c
@@ -124,3 +124,8 @@ int libxl__pci_topology_init(libxl__gc *gc,
 {
     return ERROR_NI;
 }
+
+void libxl__local_dm_preexec_restrict(libxl__gc *gc, int stderrfd)
+{
+    return;
+}
-- 
2.19.1


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