util: generate a persistent system token

When creating the system identity set the system token. The system
token is currently stored in a local path

   /var/run/libvirt/common/system.token

Obviously with only traditional UNIX DAC in effect, this is largely
security through obscurity, if the client is running at the same
privilege level as the daemon. It does, however, reliably distinguish
an unprivileged client from the system daemons.

With a MAC system like SELinux though, or possible use of containers,
access can be further restricted.

A possible future improvement for Linux would be to populate the
kernel keyring with a secret for libvirt daemons to share.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

src/libvirt_private.syms

@@ -2396,6 +2396,7 @@ virHostGetBootTime;
 
 
 # util/viridentity.h
+virIdentityEnsureSystemToken;
 virIdentityGetCurrent;
 virIdentityGetGroupName;
 virIdentityGetParameters;

src/util/viridentity.c

@@ -22,21 +22,27 @@
 #include <config.h>
 
 #include <unistd.h>
+#include <fcntl.h>
 #if WITH_SELINUX
 # include <selinux/selinux.h>
 #endif
 
+#define LIBVIRT_VIRIDENTITYPRIV_H_ALLOW
+
 #include "internal.h"
 #include "viralloc.h"
 #include "virerror.h"
-#include "viridentity.h"
+#include "viridentitypriv.h"
 #include "virlog.h"
 #include "virobject.h"
+#include "virrandom.h"
 #include "virthread.h"
 #include "virutil.h"
 #include "virstring.h"
 #include "virprocess.h"
 #include "virtypedparam.h"
+#include "virfile.h"
+#include "configmake.h"
 
 #define VIR_FROM_THIS VIR_FROM_IDENTITY
 
@@ -55,6 +61,7 @@ struct _virIdentity {
 G_DEFINE_TYPE(virIdentity, vir_identity, G_TYPE_OBJECT)
 
 static virThreadLocal virIdentityCurrent;
+static char *systemToken;
 
 static void virIdentityFinalize(GObject *obj);
 
@@ -73,6 +80,9 @@ static int virIdentityOnceInit(void)
         return -1;
     }
 
+    if (!(systemToken = virIdentityEnsureSystemToken()))
+        return -1;
+
     return 0;
 }
 
@@ -144,6 +154,101 @@ int virIdentitySetCurrent(virIdentity *ident)
 }
 
 
+#define TOKEN_BYTES 16
+#define TOKEN_STRLEN (TOKEN_BYTES * 2)
+
+static char *
+virIdentityConstructSystemTokenPath(void)
+{
+    g_autofree char *commondir = NULL;
+    if (geteuid() == 0) {
+        commondir = g_strdup(RUNSTATEDIR "/libvirt/common");
+    } else {
+        g_autofree char *rundir = virGetUserRuntimeDirectory();
+        commondir = g_strdup_printf("%s/common", rundir);
+    }
+
+    if (g_mkdir_with_parents(commondir, 0700) < 0) {
+        virReportSystemError(errno,
+                             _("Cannot create daemon common directory '%s'"),
+                             commondir);
+        return NULL;
+    }
+
+    return g_strdup_printf("%s/system.token", commondir);
+}
+
+
+char *
+virIdentityEnsureSystemToken(void)
+{
+    g_autofree char *tokenfile = virIdentityConstructSystemTokenPath();
+    g_autofree char *token = NULL;
+    VIR_AUTOCLOSE fd = -1;
+    struct stat st;
+
+    if (!tokenfile)
+        return NULL;
+
+    fd = open(tokenfile, O_RDWR|O_APPEND|O_CREAT, 0600);
+    if (fd < 0) {
+        virReportSystemError(errno,
+                             _("Unable to open system token %s"),
+                             tokenfile);
+        return NULL;
+    }
+
+    if (virSetCloseExec(fd) < 0) {
+        virReportSystemError(errno,
+                             _("Failed to set close-on-exec flag '%s'"),
+                             tokenfile);
+        return NULL;
+    }
+
+    if (virFileLock(fd, false, 0, 1, true) < 0) {
+        virReportSystemError(errno,
+                             _("Failed to lock system token '%s'"),
+                             tokenfile);
+        return NULL;
+    }
+
+    if (fstat(fd, &st) < 0) {
+        virReportSystemError(errno,
+                             _("Failed to check system token '%s'"),
+                             tokenfile);
+        return NULL;
+    }
+
+    /* Ok, we're the first one here, so we must populate it */
+    if (st.st_size == 0) {
+        if (!(token = virRandomToken(TOKEN_BYTES))) {
+            return NULL;
+        }
+        if (safewrite(fd, token, TOKEN_STRLEN) != TOKEN_STRLEN) {
+            virReportSystemError(errno,
+                                 _("Failed to write system token '%s'"),
+                                 tokenfile);
+            return NULL;
+        }
+    } else {
+        if (virFileReadLimFD(fd, TOKEN_STRLEN, &token) < 0) {
+            virReportSystemError(errno,
+                                 _("Failed to write system token '%s'"),
+                                 tokenfile);
+            return NULL;
+        }
+        if (strlen(token) != TOKEN_STRLEN) {
+            virReportSystemError(errno,
+                                 _("System token in %s was corrupt"),
+                                 tokenfile);
+            return NULL;
+        }
+    }
+
+    return g_steal_pointer(&token);
+}
+
+
 /**
  * virIdentityGetSystem:
  *

src/util/viridentitypriv.h

@@ -0,0 +1,30 @@
+/*
+ * viridentitypriv.h: helper APIs for managing user identities
+ *
+ * Copyright (C) 2012-2013 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library;  If not, see
+ * <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef LIBVIRT_VIRIDENTITYPRIV_H_ALLOW
+# error "viridentitypriv.h may only be included by viridentity.c or test suites"
+#endif /* LIBVIRT_VIRIDENTITYPRIV_H_ALLOW */
+
+#pragma once
+
+#include "viridentity.h"
+
+char *
+virIdentityEnsureSystemToken(void) G_GNUC_NO_INLINE;

tests/viridentitytest.c

@@ -25,7 +25,9 @@
 
 #include "testutils.h"
 
-#include "viridentity.h"
+#define LIBVIRT_VIRIDENTITYPRIV_H_ALLOW
+
+#include "viridentitypriv.h"
 #include "virerror.h"
 #include "viralloc.h"
 #include "virlog.h"
@@ -36,6 +38,13 @@
 
 VIR_LOG_INIT("tests.identitytest");
 
+char *
+virIdentityEnsureSystemToken(void)
+{
+    return g_strdup("3de80bcbf22d4833897f1638e01be9b2");
+}
+
+
 static int testIdentityAttrs(const void *data G_GNUC_UNUSED)
 {
     g_autoptr(virIdentity) ident = virIdentityNew();