nscd: Make SELinux checks dynamic.

The SELinux team has indicated to me that glibc's SELinux checks
in nscd are not being carried out as they would expect the API
to be used today. They would like to move away from static header
defines for class and permissions and instead use dynamic checks
at runtime that provide an answer which is dependent on the runtime
status of SELinux i.e. more dynamic.

The following patch is a minimal change that moves us forward in
this direction.

It does the following:

* Stop checking for SELinux headers that define NSCD__SHMEMHOST.
  Check only for the presence or absence of the library.

* Don't encode the specific SELinux permission constants into a
  table at build time, and instead use the symbolic name for the
  permission as expected.

* Lookup the "What do we do if we don't know this permission?"
  policy and use that if we find SELinux's policy is older than
  the glibc policy e.g. we make a request for a permission that
  SELinux doesn't know about.

* Lastly, translate the class and permission and then make
  the permission check. This is done every time we lookup
  a permission, and this is the expected way to use the API.
  SELinux will optimize this for us, and we expect the network
  latencies to hide these extra library calls.

Tested on x86, x86-64, and via Fedora Rawhide since November 2013.

See:
https://sourceware.org/ml/libc-alpha/2014-04/msg00179.html

1 files changed, 68 insertions, 39 deletions

diff --git a/nscd/selinux.c b/nscd/selinux.c
index 46b0ea9..9a8a5a8 100644
--- a/nscd/selinux.c
+++ b/nscd/selinux.c
@@ -28,7 +28,6 @@
 #include <syslog.h>
 #include <unistd.h>
 #include <sys/prctl.h>
-#include <selinux/av_permissions.h>
 #include <selinux/avc.h>
 #include <selinux/flask.h>
 #include <selinux/selinux.h>
@@ -44,35 +43,31 @@
 /* Global variable to tell if the kernel has SELinux support.  */
 int selinux_enabled;
 
-/* Define mappings of access vector permissions to request types.  */
-static const access_vector_t perms[LASTREQ] =
+/* Define mappings of request type to AVC permission name.  */
+static const char *perms[LASTREQ] =
 {
-  [GETPWBYNAME] = NSCD__GETPWD,
-  [GETPWBYUID] = NSCD__GETPWD,
-  [GETGRBYNAME] = NSCD__GETGRP,
-  [GETGRBYGID] = NSCD__GETGRP,
-  [GETHOSTBYNAME] = NSCD__GETHOST,
-  [GETHOSTBYNAMEv6] = NSCD__GETHOST,
-  [GETHOSTBYADDR] = NSCD__GETHOST,
-  [GETHOSTBYADDRv6] = NSCD__GETHOST,
-  [GETSTAT] = NSCD__GETSTAT,
-  [SHUTDOWN] = NSCD__ADMIN,
-  [INVALIDATE] = NSCD__ADMIN,
-  [GETFDPW] = NSCD__SHMEMPWD,
-  [GETFDGR] = NSCD__SHMEMGRP,
-  [GETFDHST] = NSCD__SHMEMHOST,
-  [GETAI] = NSCD__GETHOST,
-  [INITGROUPS] = NSCD__GETGRP,
-#ifdef NSCD__GETSERV
-  [GETSERVBYNAME] = NSCD__GETSERV,
-  [GETSERVBYPORT] = NSCD__GETSERV,
-  [GETFDSERV] = NSCD__SHMEMSERV,
-#endif
-#ifdef NSCD__GETNETGRP
-  [GETNETGRENT] = NSCD__GETNETGRP,
-  [INNETGR] = NSCD__GETNETGRP,
-  [GETFDNETGR] = NSCD__SHMEMNETGRP,
-#endif
+  [GETPWBYNAME] = "getpwd",
+  [GETPWBYUID] = "getpwd",
+  [GETGRBYNAME] = "getgrp",
+  [GETGRBYGID] = "getgrp",
+  [GETHOSTBYNAME] = "gethost",
+  [GETHOSTBYNAMEv6] = "gethost",
+  [GETHOSTBYADDR] = "gethost",
+  [GETHOSTBYADDRv6] = "gethost",
+  [SHUTDOWN] = "admin",
+  [GETSTAT] = "getstat",
+  [INVALIDATE] = "admin",
+  [GETFDPW] = "shmempwd",
+  [GETFDGR] = "shmemgrp",
+  [GETFDHST] = "shmemhost",
+  [GETAI] = "gethost",
+  [INITGROUPS] = "getgrp",
+  [GETSERVBYNAME] = "getserv",
+  [GETSERVBYPORT] = "getserv",
+  [GETFDSERV] = "shmemserv",
+  [GETNETGRENT] = "getnetgrp",
+  [INNETGR] = "getnetgrp",
+  [GETFDNETGR] = "shmemnetgrp",
 };
 
 /* Store an entry ref to speed AVC decisions.  */
@@ -344,7 +339,16 @@ nscd_avc_init (void)
 
 
 /* Check the permission from the caller (via getpeercon) to nscd.
-   Returns 0 if access is allowed, 1 if denied, and -1 on error.  */
+   Returns 0 if access is allowed, 1 if denied, and -1 on error.
+
+   The SELinux policy, enablement, and permission bits are all dynamic and the
+   caching done by glibc is not entirely correct.  This nscd support should be
+   rewritten to use selinux_check_permission.  A rewrite is risky though and
+   requires some refactoring.  Currently we use symbolic mappings instead of
+   compile time constants (which SELinux upstream says are going away), and we
+   use security_deny_unknown to determine what to do if selinux-policy* doesn't
+   have a definition for the the permission or object class we are looking
+   up.  */
 int
 nscd_request_avc_has_perm (int fd, request_type req)
 {
@@ -354,6 +358,33 @@ nscd_request_avc_has_perm (int fd, request_type req)
   security_id_t ssid = NULL;
   security_id_t tsid = NULL;
   int rc = -1;
+  security_class_t sc_nscd;
+  access_vector_t perm;
+  int avc_deny_unknown;
+
+  /* Check if SELinux denys or allows unknown object classes
+     and permissions.  It is 0 if they are allowed, 1 if they
+     are not allowed and -1 on error.  */
+  if ((avc_deny_unknown = security_deny_unknown ()) == -1)
+    dbg_log (_("Error querying policy for undefined object classes "
+	       "or permissions."));
+
+  /* Get the security class for nscd.  If this fails we will likely be
+     unable to do anything unless avc_deny_unknown is 0.  */
+  sc_nscd = string_to_security_class ("nscd");
+  if (perm == 0 && avc_deny_unknown == 1)
+    dbg_log (_("Error getting security class for nscd."));
+
+  /* Convert permission to AVC bits.  */
+  perm = string_to_av_perm (sc_nscd, perms[req]);
+  if (perm == 0 && avc_deny_unknown == 1)
+      dbg_log (_("Error translating permission name "
+		 "\"%s\" to access vector bit."), perms[req]);
+
+  /* If the nscd security class was not found or perms were not
+     found and AVC does not deny unknown values then allow it.  */
+  if ((sc_nscd == 0 || perm == 0) && avc_deny_unknown == 0)
+    return 0;
 
   if (getpeercon (fd, &scon) < 0)
     {
@@ -372,15 +403,13 @@ nscd_request_avc_has_perm (int fd, request_type req)
       goto out;
     }
 
-#ifndef NSCD__GETSERV
-  if (perms[req] == 0)
-    {
-      dbg_log (_("compile-time support for database policy missing"));
-      goto out;
-    }
-#endif
-
-  rc = avc_has_perm (ssid, tsid, SECCLASS_NSCD, perms[req], &aeref, NULL) < 0;
+  /* The SELinux API for avc_has_perm conflates access denied and error into
+     the return code -1, while nscd_request_avs_has_perm has distinct error
+     (-1) and denied (1) return codes. We map the avc_has_perm access denied or
+     error into an access denied at the nscd interface level (we do accurately
+     report error for the getpeercon, getcon, and avc_context_to_sid interfaces
+     used above).  */
+  rc = avc_has_perm (ssid, tsid, sc_nscd, perm, &aeref, NULL) < 0;
 
 out:
   if (scon)