CVE-2021-4034

A local privilege escalation vulnerability was found on polkit's pkexec utility. The pkexec application is a setuid tool designed to allow unprivileged users to run commands as privileged users according predefined policies. The current version of pkexec doesn't handle the calling parameters count correctly and ends trying to execute environment variables as commands. An attacker can leverage this by crafting environment variables in such a way it'll induce pkexec to execute arbitrary code. When successfully executed the attack can cause a local privilege escalation given unprivileged users administrative rights on the target machine.

The following steps can be ran as mitigation on systems that doesn't have the Secure Boot feature enabled:

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

2) Install polkit debug info:

    debuginfo-install polkit
    

3) Create the following systemtap script, and name it pkexec-block.stp:

    probe process("/usr/bin/pkexec").function("main")  {
        if (cmdline_arg(1) == "")
                        raise(9);
}

4) Load the systemtap module into the running kernel:

    stap -g -F -m stap_pkexec_block pkexec_block.stp
    

5) Ensure the module is loaded:

     lsmod | grep -i stap_pkexec_block
stap_pkexec_block     434176  0

6) Once polkit package was updated to the version containing the fix, the systemtap generated kernel module can be removed by running:

    rmmod stap_pkexec_block
    

This mitigation doesn't work for Secure Boot enabled system as SystemTap would require an external compiling server to be able to sign the generated kernel module
with a key enrolled into the Kernel's keyring.