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=============================================================================== Trusted Path Execution (TPE) Linux Kernel Module =============================================================================== About this module Trusted Path Execution is a security feature that denies users from executing programs that are not owned by root, or are writable. This closes the door on a whole category of exploits where a malicious user tries to execute his or her own code to attack the system. Since this module doesn't use any kind of ACLs, it works out of the box with no configuration. It isn't complicated to test or deploy to current production systems. Just install it and you're done! =============================================================================== WARNING! Use at your own risk! Although stable on tested systems, I have never used your system before. I can not guarantee that it won't crash your system, melt your hard drive, get you fired from your job, and/or any other horrible event. Use at your own risk. =============================================================================== Supported Kernels This has been tested on the following systems (x86, both 32 and 64bit): - RHEL/CentOS 5 (linux-2.6.18) - 32bit no longer supported - RHEL/CentOS 6 (linux-2.6.32) - RHEL/CentOS 7 (linux-3.10.0) - RHEL/CentOS Xen, both xenU (el5) and pvops (el6) - RHEL/CentOS KVM - Ubuntu 12.04 LTS (linux-3.2.0) - Ubuntu 14.04 LTS (linux-3.13.0) This module *should* work on most linux kernels version 2.6 and above, but has only been verified on the above systems. If you get a compile error or a kernel oops, please contact this module's author. =============================================================================== Features * Trusted Path Execution; deny execution of non-root owned or writable binaries $ gcc -o exploit exploit.c $ chmod 755 exploit $ ./exploit -bash: ./exploit: Permission denied $ dmesg | tail -n1 [tpe] Denied untrusted exec of /home/corey/exploit (uid:500) by /bin/bash (uid:500), parents: /usr/sbin/sshd (uid:500), /usr/sbin/sshd (uid:0), /sbin/init (uid:0). Deny reason: directory uid not trusted Extras, also ported from grsecurity (but not tpe related): * Restrict non-root users from viewing loaded kernel modules $ cat /proc/modules cat: /proc/modules: Operation not permitted * Restrict non-root users from viewing the kernel symbol table $ cat /proc/kallsyms cat: /proc/kallsyms: Operation not permitted * Restrict non-root users from viewing processes they don't own $ ps auxf USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND corey 22450 0.0 0.7 97780 1804 ? S 14:12 0:00 sshd: corey @pts/1 corey 22451 0.0 0.7 108288 1884 pts/1 Ss 14:12 0:00 \_ -bash corey 22474 0.0 0.4 110184 1044 pts/1 R+ 14:12 0:00 \_ ps auxf See the "Configuration" section below for how to enable these extras. =============================================================================== Installation See the INSTALL file for installation instructions. =============================================================================== FAQ See the FAQ file for frequently asked questions. =============================================================================== Configuration Although most people will find they don't need to change the default values, you have the option to configure various things in this module at runtime using the sysctl interface to tighten or lax the TPE restrictions. You can see the values in this proc directory: /proc/sys/tpe/ softmode - log what would be denied but don't actually deny. default off strict - enforce some TPE features even on trusted users. default on check_file - check file owner/mode in addition to directory. default on group_writable - check if the file/directory is group writable. default on kill - kill the offending process and its parent when it gets denied execution from TPE, unless it's root. default off log - whether to log denied execs to the ring buffer. default on log_max - maximun parent processes in a single log entry. default 50 log_floodburst - number of log entries before logging is disabled. default 5 log_floodtime - seconds until re-enabling logging after floodburst. default 5 paranoid - enforce the trusted path restrictions root too. default off hardcoded_path - use with caution! a list of directories, separated by colons, that the trusted path will be restricted to; nothing outside this path may be executed/mmaped. default to empty (off) trusted_gid - gid of trusted users who TPE is not enforced. default 0 (off) trusted_invert - changes what "trusted_gid" means; if set, users in the gid will be the only ones whom TPE is enforced. default 0 (off) admin_gid - files belonging to this group are treated as if they're owned by root; TPE is not enforced on them. default 0 (off) dmz_gid - users in this gid can't exec anything at all. default 0 (off) lock - when enabled, these sysctl entries can no longer be changed. extras/ - directory for additional protections that aren't TPE related. These protections are all off by default, and are as follows: lsmod - denies non-root users from viewing loaded kernel modules proc_kallsyms - denies non-root users from viewing /proc/kallsyms ps - denies non-root users from viewing processes they don't own ps_gid - gid of users who aren't restricted by ps. default 0 (off) restrict_setuid - users not in the trusted_gid are denied calls to setuid() harden_symlink - denies non-root users from following symlinks to files owned by a different user (like grsecurity's GRKERNSEC_SYMLINKOWN) harden_hardlinks - denies non-root users from creating hardlinks to files owned by a different user, denies non-root users from hardlinking to special files (device nodes, etc) Edit the /etc/sysctl.d/tpe.conf file to change the defaults of these various features. I highly recommend that you turn on the "extras" where possible. Some sysctl options my not exist due to them not being possible on the kernel, or the feature was pushed upstream. Notable examples are the link setuid and link hardening. We used to do dmesg_restrict too, but have since removed it due to it being in recent versions of all the supported kernels. =============================================================================== Compatibility Issues Test this module before deploying to a critical system, especially if you're not using a kernel in the above "Supported Kernels" list. Trusted Path Execution will cause some programs to stop functioning correctly. If it doesn't result in a kernel BUG or other stack-trace to show up in dmesg, it's the program that needs fixing, not this module. A way to work around this problem is adding the uid of the user these broken programs run as to the "Trusted GID" group, so TPE is not enforced on them. You can set this group id in /proc/sys/tpe/trusted_gid List of programs known to fail while TPE is active for the user running it: * wine * dropbox This module will not work on systems where loadable kernel module support is disabled at compile time (CONFIG_MODULES not being set). This module requires many LSM hooks in order to work, and will error out when security is disabled at compile time (CONFIG_SECURITY not being set). If you custom compile your own kernel and want TPE, consider using grsecurity instead of this module. This module may conflict with ksplice, but I've never actually tested that theory. If you use that software, let me know whether or not you run into problems when using this module. =============================================================================== TPE Coverage This code was stared as a "proof of concept" and turned into a usable security tool. It may not cover all entry points that TPE needs to cover. If you find a way to bypass the trusted path, let me know, and I'll update this code. =============================================================================== BUGS There are currently no known bugs. If you discover one, please report it to the author. If your system crashes as a result of TPE, consider setting the following: sudo sysctl kernel.panic_on_oops=0 This will give you a chance to capture the kernel errors in your system logs. =============================================================================== Acknowledgements - memset http://memset.wordpress.com/2010/12/03/syscall-hijacking-kernel-2-6-systems/ With that I learnt how to hijack system calls and bypass the "general protection fault" which has previously been blocking me from doing this. - Eugene Shatokhin There were a few not-so-minor bugs in this module, and I asked this question on Stack Overflow: http://stackoverflow.com/questions/6434701/having-trouble-wrapping-functions-in-the-linux-kernel Eugene helped me out and shared code which is now in use by this module, which has made it stable. - Brad Spengler - http://grsecurity.net/ Trusted Path Execution, and the "extra" features in this module are all features of grsecurity, and I originally pulled code from that project to make this module. This module's TPE code has since evolved to cover much more than grsecurity's TPE feature; however, anything in this module beyond grsecurity's TPE can be done with grsecurity's RBAC system, so it is no substitute. =============================================================================== About the Author website: http://cormander.com/ github: https://github.com/cormander/ email: corman /AT/ cormander /DOT/ com ===============================================================================