buffer overflow in ld.so leading to privilege escalation
CVE: CVE-2023-4911
Component: glibc
CWE: CWE-122
Severity: Important
CVSS v3: 7.9/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Release date: 2023-10-03T17:00:00+00:00

A buffer overflow was discovered in the GNU C Library's dynamic loader ld.so while processing the GLIBC_TUNABLES environment variable. This issue could allow a local attacker to use maliciously crafted GLIBC_TUNABLES environment variables when launching binaries with SUID permission to execute code with elevated privileges.

Statement: This vulnerability was introduced in glibc version 2.34. RHEL-8 ships glibc 2.28, which is not originally affected by this vulnerability. However, the commit that introduced this vulnerability was backported to RHEL-8.5, making this version and onward vulnerable. RHEL-8.4 and older are not affected by this vulnerability.

Remediation:
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

For the update to take effect, all services linked to the glibc library must be restarted, or the system rebooted.
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/2974891
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

For the update to take effect, all services linked to the glibc library must be restarted, or the system rebooted.
Before applying this update, make sure all previously released errata
relevant to your system have been applied.

For details on how to apply this update, refer to:

https://access.redhat.com/articles/11258
Before applying this update, make sure all previously released errata
relevant to your system have been applied.

For details on how to apply this update, refer to:

https://access.redhat.com/articles/11258
For customers who cannot update immediately and do not have Secure Boot feature enabled, the issue can be mitigated using the provided SystemTap script with the following steps.  When enabled, any setuid program invoked with GLIBC_TUNABLES in the environment will be terminated immediately.  To invoke the setuid program, users will then have to unset or clear the GLIBC_TUNABLES envvar, e.g. `GLIBC_TUNABLES= sudo` . 

Note that these mitigation steps will need to be repeated if the system is rebooted.

1)    Install required systemtap packages and dependencies as per - https://access.redhat.com/solutions/5441


2)    Create the following systemtap script, and name it stap_block_suid_tunables.stp:
    ~~~
function has_tunable_string:long()
{
  name = "GLIBC_TUNABLES"

  mm = @task(task_current())->mm;
  if (mm)
    {
      env_start = @mm(mm)->env_start;
      env_end = @mm(mm)->env_end;

      if (env_start != 0 && env_end != 0)
        while (env_end > env_start)
          {
            cur = user_string(env_start, "");
            env_name = tokenize(cur, "=");
      
            if (env_name == name && tokenize("", "") != "")
              return 1;
            env_start += strlen (cur) + 1
          }
    }

  return 0;
}

probe process("/lib*/ld*.so*").function("__tunables_init")
{
  atsecure = 0;
  /* Skip processing if we can't read __libc_enable_secure, e.g. core dump
     handler (systemd-cgroups-agent and systemd-coredump).  */
  try { atsecure = @var("__libc_enable_secure"); }
  catch { printk (4, sprintf ("CVE-2023-4911: Skipped check: %s (%d)", execname(), pid())); }
  if (atsecure && has_tunable_string ())
    raise (9);
}
~~~

3) Load the systemtap module into the running kernel:
    ~~~
    stap -g -F -m stap_block_suid_tunables stap_block_suid_tunables.stp
    ~~~

4) Ensure the module is loaded:
    ~~~
     lsmod | grep -i stap_block_suid_tunables
stap_block_suid_tunables     249856  0
~~~

5) Once the glibc package is updated to the version containing the fix, the systemtap generated kernel module can be removed by running:
    ~~~
    rmmod stap_block_suid_tunables
    ~~~

If Secure Boot is enabled on a system, the SystemTap module must be signed. An external compiling server can be used to sign the generated kernel module with a key enrolled into the kernel's keyring or starting with SystemTap 4.7 you can sign a module without a compile server. See further information here - https://www.redhat.com/sysadmin/secure-boot-systemtap