nss: Re-enable NSS module loading after chroot [BZ #27389]

The glibc 2.33 release enabled /etc/nsswitch.conf reloading,
and to prevent potential security issues like CVE-2019-14271
the re-loading of nsswitch.conf and all mdoules was disabled
when the root filesystem changes (see bug 27077).

Unfortunately php-lpfm and openldap both require the ability
to continue to load NSS modules after chroot. The packages
do not exec after the chroot, and so do not cause the
protections to be reset. The only solution is to re-enable
only NSS module loading (not nsswitch.conf reloading) and so
get back the previous glibc behaviour.

In the future we may introduce a way to harden applications
so they do not reload NSS modules once the root filesystem
changes, or that only files/dns are available pre-loaded
(or builtin).

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit 58673149f3)

nss/nss_database.c

@@ -402,7 +402,6 @@ nss_database_check_reload_and_get (struct nss_database_state *local,
       atomic_store_release (&local->data.reload_disabled, 1);
       *result = local->data.services[database_index];
       __libc_lock_unlock (local->lock);
-      __nss_module_disable_loading ();
       return true;
     }
   local->root_ino = str.st_ino;

nss/tst-reload2.c

@@ -26,6 +26,7 @@
 #include <pwd.h>
 #include <grp.h>
 #include <unistd.h>
+#include <netdb.h>
 
 #include <support/support.h>
 #include <support/check.h>
@@ -48,7 +49,7 @@ static const char *group_4[] = {
   "alpha", "beta", "gamma", "fred", NULL
 };
 
-static struct group group_table_data[] =
+static struct group group_table_data1[] =
   {
    GRP (4),
    GRP_LAST ()
@@ -58,7 +59,7 @@ void
 _nss_test1_init_hook (test_tables *t)
 {
   t->pwd_table = pwd_table1;
-  t->grp_table = group_table_data;
+  t->grp_table = group_table_data1;
 }
 
 static struct passwd pwd_table2[] =
@@ -68,10 +69,21 @@ static struct passwd pwd_table2[] =
    PWD_LAST ()
   };
 
+static const char *group_5[] = {
+  "fred", NULL
+};
+
+static struct group group_table_data2[] =
+  {
+   GRP (5),
+   GRP_LAST ()
+  };
+
 void
 _nss_test2_init_hook (test_tables *t)
 {
   t->pwd_table = pwd_table2;
+  t->grp_table = group_table_data2;
 }
 
 static int
@@ -79,6 +91,7 @@ do_test (void)
 {
   struct passwd *pw;
   struct group *gr;
+  struct hostent *he;
   char buf1[PATH_MAX];
   char buf2[PATH_MAX];
 
@@ -99,7 +112,9 @@ do_test (void)
     TEST_COMPARE (pw->pw_uid, 1234);
 
   /* This just loads the test2 DSO.  */
-  gr = getgrnam ("name4");
+  gr = getgrgid (5);
+  TEST_VERIFY (gr != NULL);
+
 
   /* Change the root dir.  */
 
@@ -114,15 +129,21 @@ do_test (void)
   if (pw)
     TEST_VERIFY (pw->pw_uid != 2468);
 
-  /* The "files" DSO should not be loaded.  */
-  gr = getgrnam ("test3");
-  TEST_VERIFY (gr == NULL);
-
   /* We should still be using the old configuration.  */
   pw = getpwnam ("test1");
   TEST_VERIFY (pw != NULL);
   if (pw)
     TEST_COMPARE (pw->pw_uid, 1234);
+  gr = getgrgid (5);
+  TEST_VERIFY (gr != NULL);
+  gr = getgrnam ("name4");
+  TEST_VERIFY (gr == NULL);
+
+  /* hosts in the outer nsswitch is files; the inner one is test1.
+     Verify that we're still using the outer nsswitch *and* that we
+     can load the files DSO. */
+  he = gethostbyname ("test2");
+  TEST_VERIFY (he != NULL);
 
   return 0;
 }

nss/tst-reload2.root/etc/hosts

@@ -0,0 +1 @@
+1.2.3.4 test1

nss/tst-reload2.root/etc/nsswitch.conf

@@ -1,2 +1,3 @@
 passwd: test1
 group: test2
+hosts: files

nss/tst-reload2.root/subdir/etc/hosts

@@ -0,0 +1 @@
+1.2.3.4 test2

nss/tst-reload2.root/subdir/etc/nsswitch.conf

@@ -1,2 +1,3 @@
 passwd: test2
 group: files
+hosts: test1