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SELinux Administration
Pax Dickinson
Hi, and welcome to the third in a series of articles on Security Enhanced Linux. My first SELinux article detailed the background of SELinux, while my second article in the series discussed how SELinux makes access decisions. This week, I'll talk about how an SELinux system differs from a standard Linux system in terms of administration. Most of what you already know about Linux system administration will still apply to an SELinux system, but there are some additions and changes that are critical to understand when using SELinux.
Permissive mode vs. Enforcing mode
There will be times when you have run into difficulty and need to determine whether your problem stems from SELinux or not. For just this eventuality, SELinux includes the capability of setting its mode from enforcing to permissive and back again. Enforcing mode is just what it sounds like, a mode that allows SELinux to enforce policy access decisions. This is the standard operating mode of SELinux. Permissive mode, on the other hand, is a mode designed for development and troubleshooting. It will still check the security policy to see whether an attempted operation should be allowed, and log denials to the system logs, but it will not actually deny any operation.
To change into permissive mode, be sure you are logged in to the sysadm_r role (see my previous article for details). Issuing a setenforce 0 command will put the system into permissive mode, while a setenforce 1 command will return you to enforcing mode. To determine the current SELinux mode, use the getenforce command.
If you want to completely disable SELinux, you can pass selinux=0 to the kernel command line at startup, but this is not advisable since it disables SELinux entirely and any new files will not be labeled with the correct file context, forcing you to relabel when you re-enable SELinux. It's better to use permissive mode, and you can set your system to always start up in permissive mode by editing your /etc/selinux/config file.
File Context Labeling
SELinux file types are attched to each file on your SELinux system using extended file attributes. The use of these attributes is integral and required by SELinux, and has some system administration ramifications you should be aware of.
When formatting a new filesystem for use with SELinux, you must use a filesystem that supports these extended attributes. The ext2 and ext3 filesystems support extended attributes, and the xfs filesystem also is known to work, but reiserfs does not currently include extended attribute support.
When backing up files on an SELinux system, you need to use a backup method that is aware of and backs up these extended attributes. For example, the standard tar command will not back them up, so you need to use star as a substitute. star is an extension of the tar command, so you shouldn't run into serious problems here, but this could have ramifications with any backup scripts you may have written that call the tar command.
A common cause of SELinux problems is caused by mislabeled files. If you run into strange errors or see files that are mislabeled, the best, most reliable way of fixing them is to issue a touch /.autorelabel command followed by a reboot. This will trigger a relabel upon startup of the system, before files are opened and services are started. The restorecon command can also be used to restore files to their proper context, but it won't change the running context of processes that were launched by a mislabeled binary, so you may still run into problems.
The chcon command can be used to change the context of a file, but if the file has a default context set in the policy it will be reset to that default if the entire filesystem is relabeled. chcon is most useful for testing new file contexts before making a change permanent in the policy, if your system depends on contexts set using chcon you may run into trouble if you ever need to perform a global relabeling.
Finally, it is important to be aware of the differences between copying and moving files using the cp or mv commands. When moving a file using mv, the destination file will retain its original context. When copying a file using cp, the file will inherit a new context based on the destination directory it was copied to. This is an important distinction that can result in trouble if it is overlooked.
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Introduction: Buffer Overflow Vulnerabilities - Buffer overflows are a leading type of security vulnerability. This paper explains what a buffer overflow is, how it can be exploited, and what countermeasures can be taken to prevent the use of buffer overflow vulnerabilities.
Getting to Know Linux Security: File Permissions - Welcome to the first tutorial in the 'Getting to Know Linux Security' series. The topic explored is Linux file permissions. It offers an easy to follow explanation of how to read permissions, and how to set them using chmod. This guide is intended for users new to Linux security, therefore very simple. If the feedback is good, I'll consider creating more complex guides for advanced users. Please let us know what you think and how these can be improved.
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Thank you for reading the LinuxSecurity.com weekly security newsletter. The purpose of this document is to provide our readers with a quick summary of each week's most relevant Linux security headline.
| Debian | ||
| Debian: New OpenSSL 0.9.6 packages fix cryptographic weakness | ||
| 4th, November, 2005
Updated package. advisories/debian/debian-new-openssl-096-packages-fix-cryptographic-weakness |
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| Debian: New OpenSSL packages fix cryptographic weakness | ||
| 4th, November, 2005
Updated package. advisories/debian/debian-new-openssl-packages-fix-cryptographic-weakness-46955 |
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| Debian: New thttpd packages fix insecure temporary file | ||
| 4th, November, 2005
Updated package. advisories/debian/debian-new-thttpd-packages-fix-insecure-temporary-file |
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| Debian: New Horde3 packages fix insecure default installation | ||
| 7th, November, 2005
Updated package. advisories/debian/debian-new-horde3-packages-fix-insecure-default-installation |
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| Debian: New OpenVPN packages fix several vulnerabilities | ||
| 7th, November, 2005
Updated package. advisories/debian/debian-new-openvpn-packages-fix-several-vulnerabilities |
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| Debian: New squid packages fix regression | ||
| 7th, November, 2005
Updated package. advisories/debian/debian-new-squid-packages-fix-regression |
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| Debian: New chmlib packages fix several vulnerabilities | ||
| 7th, November, 2005
Updated package. advisories/debian/debian-new-chmlib-packages-fix-several-vulnerabilities |
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| Debian: New ClamAV packages fix several vulnerabilities | ||
| 7th, November, 2005
Updated package. advisories/debian/debian-new-clamav-packages-fix-several-vulnerabilities-27806 |
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| Debian: New OpenSSL packages fix cryptographic weakness | ||
| 7th, November, 2005
Updated package. advisories/debian/debian-new-openssl-packages-fix-cryptographic-weakness-46955 |
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| Debian: New enigmail packages fix information disclosure | ||
| 8th, November, 2005
Updated package. advisories/debian/debian-new-enigmail-packages-fix-information-disclosure |
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| Debian: New libungif4 packages fix several vulnerabilities | ||
| 9th, November, 2005
Updated package. advisories/debian/debian-new-libungif4-packages-fix-several-vulnerabilities |
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| Debian: New gpsdrive packages fix arbitrary code execution | ||
| 9th, November, 2005
Updated package. advisories/debian/debian-new-gpsdrive-packages-fix-arbitrary-code-execution |
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| Debian: New awstats packages fix arbitrary command execution | ||
| 10th, November, 2005
Updated package. advisories/debian/debian-new-awstats-packages-fix-arbitrary-command-execution-97479 |
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| Debian: New kdelibs packages fix backup file information leak | ||
| 10th, November, 2005
Updated package. advisories/debian/debian-new-kdelibs-packages-fix-backup-file-information-leak-15398 |
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| Gentoo | ||
| Gentoo: giflib Multiple vulnerabilities | ||
| 4th, November, 2005
giflib may dereference NULL or write out of bounds when processing malformed images, potentially resulting in Denial of Service or arbitrary code execution. |
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| Gentoo: ClamAV Multiple vulnerabilities | ||
| 6th, November, 2005
ClamAV has many security flaws which make it vulnerable to remote execution of arbitrary code and a Denial of Service. |
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| Gentoo: GNUMP3d Directory traversal and XSS vulnerabilities | ||
| 6th, November, 2005
GNUMP3d is vulnerable to directory traversal and cross-site scripting attacks that may result in information disclosure or the compromise of a browser. |
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| Gentoo: fetchmail Password exposure in fetchmailconf | ||
| 6th, November, 2005
fetchmailconf fails to properly handle file permissions, temporarily exposing sensitive information to other local users. |
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| Gentoo: OpenVPN Multiple vulnerabilities | ||
| 6th, November, 2005
The OpenVPN client is potentially vulnerable to the execution of arbitrary code and the OpenVPN server is vulnerable to a Denial of Service issue. |
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| Gentoo: QDBM, ImageMagick, GDAL RUNPATH issues | ||
| 8th, November, 2005
Multiple packages suffer from RUNPATH issues that may allow users in the "portage" group to escalate privileges. |
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| Gentoo: libgda Format string vulnerabilities | ||
| 8th, November, 2005
Two format string vulnerabilities in libgda may lead to the execution of arbitrary code. |
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| Mandriva | ||
| Mandriva: Updated mandriva-release packages provide updated information | ||
| 7th, November, 2005
The updated mandriva-release packages provides a fixed CREDITS file. |
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| Mandriva: Updated clamav packages fix multiple vulnerabilities | ||
| 7th, November, 2005
Updated package. |
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| Mandriva: Updated openvpn packages fix multiple vulnerabilities | ||
| 8th, November, 2005
Updated package. |
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| Mandriva: Updated scim-qtimm packages fix incorrect requires for x86_64 | ||
| 9th, November, 2005
Due to a bug in the RPM requires for the scim-qtimm package, it was only installable on i586 platforms, and not on x86_64 due to differences in the naming for libqt3 (vs. lib64qt3). This update corrects the requires allowing the package to be installed on Mandriva Linux 2006/x86_64. |
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| Mandriva: Updated e2fsprogs packages fix segfault | ||
| 9th, November, 2005
The mklost+found program was segfaulting on Mandriva Linux 2006. This update corrects the problem. |
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| Mandriva: Updated ldetect-lst packages provide updated PCI information | ||
| 9th, November, 2005
The updated ldetect-lst packages provide five new PCI modem definitions in the hardware database. |
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| Mandriva: Updated drakxtools packages fix various bugs | ||
| 9th, November, 2005
The updated ldetect-lst packages provide five new PCI modem definitions in the hardware database. |
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| Mandriva: Updated libungif packages fix various vulnerabilities | ||
| 9th, November, 2005
Several bugs have been discovered in the way libungif decodes GIF images. |
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| Mandriva: Updated emacs packages fix Lisp vulnerability | ||
| 9th, November, 2005
Emacs 21.2 does not prompt or warn the user before executing Lisp code in the local variables section of a text file, which allows user-complicit attackers to execute arbitrary commands, as demonstrated using the mode-name variable. |
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| Mandriva: Updated fetchmail packages fixes fetchmailconf vulnerability | ||
| 9th, November, 2005
Thomas Wolff and Miloslav Trmac discovered a race condition in the fetchmailconf program. |
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| Mandriva: Updated w3c-libwww packages fixes DoS vulnerability. | ||
| 9th, November, 2005
Sam Varshavchik discovered the HTBoundary_put_block function in HTBound.c for W3C libwww (w3c-libwww) allows remote servers to cause a denial of service (segmentation fault) via a crafted multipart/byteranges MIME message that triggers an out-of-bounds read. |
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| Mandriva: Updated drakxtools packages fix various bugs | ||
| 9th, November, 2005
Updated package. |
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| Red Hat | ||
| RedHat: Important: libungif security update | ||
| 3rd, November, 2005
Updated libungif packages that fix two security issues are now available. This update has been rated as having important security impact by the Red Hat Security Response Team. advisories/red-hat/redhat-important-libungif-security-update-RHSA-2005-828-01 |
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| RedHat: Critical: flash-plugin security update | ||
| 9th, November, 2005
Updated Macromedia Flash Player packages that fix a security issue are now available. This update has been rated as having critical security impact by the Red Hat Security Response Team. advisories/red-hat/redhat-critical-flash-plugin-security-update-73664 |
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