Un-disable nss module loading after chroot [27389]

In response to upstream use cases, we're re-enabling this
for the time being, despite the potential security issues.

On 2/19/21 4:28 PM, DJ Delorie via Libc-alpha wrote:
> In response to upstream use cases, we're re-enabling this
> for the time being, despite the potential security issues.

Please post v2 with updated commit message.

Suggested commit message:

nss: Re-enable NSS module loading after chroot (Bug 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).


> diff --git a/nss/nss_database.c b/nss/nss_database.c
> index 162800cda3..9ff3bb6ffb 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 ();

OK.

>        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);

OK.

>  
>    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

OK.

> 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

OK.

>