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>

6 files changed, 32 insertions, 8 deletions

diff --git a/nss/nss_database.c b/nss/nss_database.c
index e1bef6bd75..fb72d0cc03 100644
--- a/nss/nss_database.c
+++ b/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;
diff --git a/nss/tst-reload2.c b/nss/tst-reload2.c
index 5dae16b4f0..5ecb032e9f 100644
--- a/nss/tst-reload2.c
+++ b/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;
 }
diff --git a/nss/tst-reload2.root/etc/hosts b/nss/tst-reload2.root/etc/hosts
new file mode 100644
index 0000000000..bbd9e494ef
--- /dev/null
+++ b/nss/tst-reload2.root/etc/hosts
@@ -0,0 +1 @@
+1.2.3.4 test1
diff --git a/nss/tst-reload2.root/etc/nsswitch.conf b/nss/tst-reload2.root/etc/nsswitch.conf
index 570795ae22..688a589519 100644
--- a/nss/tst-reload2.root/etc/nsswitch.conf
+++ b/nss/tst-reload2.root/etc/nsswitch.conf
@@ -1,2 +1,3 @@
 passwd: test1
 group: test2
+hosts: files
diff --git a/nss/tst-reload2.root/subdir/etc/hosts b/nss/tst-reload2.root/subdir/etc/hosts
new file mode 100644
index 0000000000..0a2cbd4337
--- /dev/null
+++ b/nss/tst-reload2.root/subdir/etc/hosts
@@ -0,0 +1 @@
+1.2.3.4 test2
diff --git a/nss/tst-reload2.root/subdir/etc/nsswitch.conf b/nss/tst-reload2.root/subdir/etc/nsswitch.conf
index f1d73f8765..fea271869e 100644
--- a/nss/tst-reload2.root/subdir/etc/nsswitch.conf
+++ b/nss/tst-reload2.root/subdir/etc/nsswitch.conf
@@ -1,2 +1,3 @@
 passwd: test2
 group: files
+hosts: test1