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Re: [libvirt] [PATCH 4/3] Control LXC capabilities



This patch updates the LXC driver to make use of libcap-ng for managing
process capabilities. Previously Ryota Ozaki had provided code to remove
the CAP_BOOT  capabilities inside the container, preventing host reboots.
In addition to that one, I believe we should be removing ability to
load kernel modules, change the system clock and changing audit/MAC.
So this patch also clears the following:

     CAP_SYS_MODULE, /* No kernel module loading */
     CAP_SYS_TIME, /* No changing the clock */
     CAP_AUDIT_CONTROL, /* No messing with auditing */
     CAP_AUDIT_WRITE, /* No messing with auditing */
     CAP_MAC_ADMIN, /* No messing with LSM */
     CAP_MAC_OVERRIDE, /* No messing with LSM */

We use libcap-ng's capng_updatev/apply functions to remove these from 
the permitted, inheritable, effective and bounding sets. Then we use
capng_lock to set NOROOT and NOROOT_LOCKED in the process securebits
to prevent them ever being re-acquired.

The other thing I realized is that the 'libvirt_lxc' controller process
does not need to keep any capabilities at all once it has spawned the 
container process, since all its doing is forwarding I/O between 2 open
file descripts. So I also clear all capabilities from that. We should
probably make it chuid/gid to a non-root user in future too. 

 lxc_container.c  |   66 +++++++++++++++++++++++++++++++++++++------------------
 lxc_controller.c |   28 +++++++++++++++++++++++
 2 files changed, 73 insertions(+), 21 deletions(-)


Regards,
Daniel

diff -r 7e766489c4a2 src/lxc_container.c
--- a/src/lxc_container.c	Tue Jun 23 11:13:45 2009 +0100
+++ b/src/lxc_container.c	Tue Jun 23 11:54:10 2009 +0100
@@ -41,8 +41,9 @@
 /* For MS_MOVE */
 #include <linux/fs.h>
 
-#include <sys/prctl.h>
-#include <linux/capability.h>
+#if HAVE_CAPNG
+#include <cap-ng.h>
+#endif
 
 #include "virterror_internal.h"
 #include "logging.h"
@@ -642,27 +643,50 @@ static int lxcContainerSetupMounts(virDo
         return lxcContainerSetupExtraMounts(vmDef);
 }
 
-static int lxcContainerDropCapabilities(virDomainDefPtr vmDef ATTRIBUTE_UNUSED)
+
+/*
+ * This is running as the 'init' process insid the container.
+ * It removes some capabilities that could be dangerous to
+ * host system, since they are not currently "containerized"
+ */
+static int lxcContainerDropCapabilities(void)
 {
-#ifdef PR_CAPBSET_DROP
-    int i;
-    const struct {
-        int id;
-        const char *name;
-    } caps[] = {
-#define ID_STRING(name) name, #name
-        { ID_STRING(CAP_SYS_BOOT) },
-    };
+#if HAVE_CAPNG
+    int ret;
 
-    for (i = 0 ; i < ARRAY_CARDINALITY(caps) ; i++) {
-        if (prctl(PR_CAPBSET_DROP, caps[i].id, 0, 0, 0)) {
-            lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
-                     _("failed to drop %s"), caps[i].name);
-            return -1;
-        }
+    capng_get_caps_process();
+
+    if ((ret = capng_updatev(CAPNG_DROP,
+                             CAPNG_EFFECTIVE | CAPNG_PERMITTED |
+                             CAPNG_INHERITABLE | CAPNG_BOUNDING_SET,
+                             CAP_SYS_BOOT, /* No use of reboot */
+                             CAP_SYS_MODULE, /* No kernel module loading */
+                             CAP_SYS_TIME, /* No changing the clock */
+                             CAP_AUDIT_CONTROL, /* No messing with auditing */
+                             CAP_AUDIT_WRITE, /* No messing with auditing */
+                             CAP_MAC_ADMIN, /* No messing with LSM */
+                             CAP_MAC_OVERRIDE, /* No messing with LSM */
+                             -1 /* sentinal */)) < 0) {
+        lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
+                 _("failed to remove capabilities %d"), ret);
+        return -1;
     }
-#else /* ! PR_CAPBSET_DROP */
-    VIR_WARN0(_("failed to drop capabilities PR_CAPBSET_DROP undefined"));
+
+    if ((ret = capng_apply(CAPNG_SELECT_BOTH)) < 0) {
+        lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
+                 _("failed to apply capabilities: %d"), ret);
+        return -1;
+    }
+
+    /* Need to prevent them regaining any caps on exec */
+    if ((ret = capng_lock()) < 0) {
+        lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
+                 _("failed to lock capabilities: %d"), ret);
+        return -1;
+    }
+
+#else
+    VIR_WARN0(_("libcap-ng support not compiled in, unable to clear capabilities"));
 #endif
     return 0;
 }
@@ -735,7 +759,7 @@ static int lxcContainerChild( void *data
         return -1;
 
     /* drop a set of root capabilities */
-    if (lxcContainerDropCapabilities(vmDef) < 0)
+    if (lxcContainerDropCapabilities() < 0)
         return -1;
 
     /* this function will only return if an error occured */
diff -r 7e766489c4a2 src/lxc_controller.c
--- a/src/lxc_controller.c	Tue Jun 23 11:13:45 2009 +0100
+++ b/src/lxc_controller.c	Tue Jun 23 11:54:10 2009 +0100
@@ -35,6 +35,10 @@
 #include <getopt.h>
 #include <sys/mount.h>
 
+#if HAVE_CAPNG
+#include <cap-ng.h>
+#endif
+
 #include "virterror_internal.h"
 #include "logging.h"
 #include "util.h"
@@ -210,6 +214,25 @@ cleanup:
     return rc;
 }
 
+
+static int lxcControllerClearCapabilities(void)
+{
+#if HAVE_CAPNG
+    int ret;
+
+    capng_clear(CAPNG_SELECT_BOTH);
+
+    if ((ret = capng_apply(CAPNG_SELECT_BOTH)) < 0) {
+        lxcError(NULL, NULL, VIR_ERR_INTERNAL_ERROR,
+                 _("failed to apply capabilities: %d"), ret);
+        return -1;
+    }
+#else
+    VIR_WARN0(_("libcap-ng support not compiled in, unable to clear capabilities"));
+#endif
+    return 0;
+}
+
 typedef struct _lxcTtyForwardFd_t {
     int fd;
     int active;
@@ -562,6 +585,11 @@ lxcControllerRun(virDomainDefPtr def,
     if (lxcContainerSendContinue(control[0]) < 0)
         goto cleanup;
 
+    /* Now the container is running, there's no need for us to keep
+       any elevated capabilities */
+    if (lxcControllerClearCapabilities() < 0)
+        goto cleanup;
+
     rc = lxcControllerMain(monitor, client, appPty, containerPty);
 
 cleanup:


-- 
|: Red Hat, Engineering, London   -o-   http://people.redhat.com/berrange/ :|
|: http://libvirt.org  -o-  http://virt-manager.org  -o-  http://ovirt.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
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