[PATCH v4 3/6] tools/dm_restrict: Ask QEMU to chroot

[George Dunlap <george.dunlap@citrix.com>]

When dm_restrict is enabled, ask QEMU to chroot into an empty directory.

* Create /var/run/qemu/root-domid (deleting the old one if it's there)
* Pass the -chroot option to QEMU

Rather than running `rm -rf` on the directory before creating it
(since there is no library function to do this), simply rmdir the
directory, relying on the fact that the previous QEMU instance, if
properly restricted, shouldn't have been able to write anything
anyway.

Suggested-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
Changes since v2:
- Style fixes
- Testing moved to a different patch

CC: Ian Jackson <ian.jackson@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Anthony Perard <anthony.perard@citrix.com>
---
 docs/designs/qemu-deprivilege.md | 12 +++++-----
 tools/libxl/libxl_dm.c           | 41 +++++++++++++++++++++++++++++++-
 2 files changed, 46 insertions(+), 7 deletions(-)

diff --git a/docs/designs/qemu-deprivilege.md b/docs/designs/qemu-deprivilege.md
index 787ae1ac7c..0395bbbb40 100644
--- a/docs/designs/qemu-deprivilege.md
+++ b/docs/designs/qemu-deprivilege.md
@@ -61,12 +61,6 @@ source tree.)
 
 '''Testing status''': 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.
-
 ## Chroot
 
 '''Description''': Qemu runs in its own chroot, such that even if it
@@ -84,6 +78,12 @@ 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
diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index 26eb16af34..ad3efcc783 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -1410,9 +1410,48 @@ static int libxl__build_device_model_args_new(libxl__gc *gc,
         }
     }
 
-    if (libxl_defbool_val(b_info->dm_restrict))
+    if (libxl_defbool_val(b_info->dm_restrict)) {
+        char *chroot_dir = GCSPRINTF("%s/qemu-root-%d",
+                                      libxl__run_dir_path(), guest_domid);
+        int r;
+        
         flexarray_append(dm_args, "-xen-domid-restrict");
 
+        /* 
+         * Run QEMU in a chroot at XEN_RUN_DIR/qemu-root-%d
+         *
+         * There is no library function to do the equivalent of `rm
+         * -rf`.  However deprivileged QEMU in theory shouldn't be
+         * able to write any files, as the chroot would be owned by
+         * root, but it would be running as an unprivileged process.
+         * So in theory, old chroots should always be empty.
+         * 
+         * rmdir the directory before attempting to create
+         * it; if it returns anything other than ENOENT, fail domain
+         * creation.
+         */
+        r = rmdir(chroot_dir);
+        if (r != 0 && errno != ENOENT) {
+            LOGED(ERROR, guest_domid,
+                  "failed to remove existing chroot dir %s", chroot_dir);
+            return ERROR_FAIL;
+        }
+        
+        for (;;) {
+            r = mkdir(chroot_dir, 0000);
+            if (!r)
+                break;
+            if (errno == EINTR) continue;
+            LOGED(ERROR, guest_domid,
+                  "failed to create chroot dir %s", chroot_dir);
+            return ERROR_FAIL;
+        }
+
+        /* Add "-chroot [dir]" to command-line */
+        flexarray_append(dm_args, "-chroot");
+        flexarray_append(dm_args, chroot_dir);
+    }
+
     if (state->saved_state) {
         /* This file descriptor is meant to be used by QEMU */
         *dm_state_fd = open(state->saved_state, O_RDONLY);
-- 
2.19.1


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