The Kernel Self-Protection project aims to make Linux more secure
Canary vs. Overflow
A stack overflow overwrites parts of the stack. Protection can be provided by a canary, a bit sequence prior to the return address that the attacker cannot predict. If the patterns no longer match, that means somebody overwrote the stack. This is a warning, in the same way that a canary warned miners of mine gas.
A skillful attacker can work around this canary protection. For example, a technique called trampolining bends existing pointers in the program so that they point to the return address. However, this approach requires considerably more effort and the use of vulnerable pointers. Similarly, an attacker might try to read out the canary value and adjust the buffer overflow appropriately, but the programmer can counteract this step by randomizing the canary.
Shadow stacks provide a similar line of defense: They keep a copy of the return address, which the attacker is unlikely to access. The process checks whether the copy matches the original before it is returned [8].
The heap, as well as the stack, is sometimes vulnerable to overflows. Attackers often take advantage of use-after-free errors: The programmer releases memory with free()
, but then uses the pointer again later on. Such errors can be corrected by more stringent checks when accessing memory areas in the kernel.
Invitation to Gamble
Attackers rely on well-known facts relating to the operating system's memory layout to inject their code (Figure 2). If the kernel randomizes the design of the addresses, attacking is like gambling. Although there are attacks like heap spraying, the overhead is growing. If both the kernel and the kernel modules are loaded at random addresses, and possibly in a random order, attacks on the kernel modules are more difficult. Similarly, random stack base addresses increase the effort required for a successful attack.
![](/var/linux_magazin/storage/images/issues/2018/209/kernel-protection/figure-2/723783-1-eng-US/Figure-2_large.png)
The more chance comes into play, the more important it is for the attacker to find out something about the memory structure or the value of canaries through information exposure. Attackers take advantage of the fact that released memory is usually not overwritten. Uninitialized variables or format string vulnerabilities also help.
To avoid this kind of information gathering, it is important to overwrite the memory immediately after releasing it. In addition, functions can be accessed via IDs and a table, analogous to the interrupt vector table, instead of directly using their addresses. This precaution means that the addresses outside the kernel are not known and are more difficult for attackers to predict.
Modular Is Cool
Attackers who have managed to inject malicious code in spite of the resistance usually look to install a rootkit, which they can do simply by loading their own kernel module into the system. To prevent this, the admin could compile the kernel statically – without module support – but a static kernel is often inconvenient, unless it is used with an embedded devices. Alternatively, the Self-Protection project suggests that only a few locally logged-in users should be allowed to load modules.
« Previous 1 2 3 Next »
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
![Learn More](https://www.linux-magazine.com/var/linux_magazin/storage/images/media/linux-magazine-eng-us/images/misc/learn-more/834592-1-eng-US/Learn-More_medium.png)
News
-
NVIDIA Released Driver for Upcoming NVIDIA 560 GPU for Linux
Not only has NVIDIA released the driver for its upcoming CPU series, it's the first release that defaults to using open-source GPU kernel modules.
-
OpenMandriva Lx 24.07 Released
If you’re into rolling release Linux distributions, OpenMandriva ROME has a new snapshot with a new kernel.
-
Kernel 6.10 Available for General Usage
Linus Torvalds has released the 6.10 kernel and it includes significant performance increases for Intel Core hybrid systems and more.
-
TUXEDO Computers Releases InfinityBook Pro 14 Gen9 Laptop
Sporting either AMD or Intel CPUs, the TUXEDO InfinityBook Pro 14 is an extremely compact, lightweight, sturdy powerhouse.
-
Google Extends Support for Linux Kernels Used for Android
Because the LTS Linux kernel releases are so important to Android, Google has decided to extend the support period beyond that offered by the kernel development team.
-
Linux Mint 22 Stable Delayed
If you're anxious about getting your hands on the stable release of Linux Mint 22, it looks as if you're going to have to wait a bit longer.
-
Nitrux 3.5.1 Available for Install
The latest version of the immutable, systemd-free distribution includes an updated kernel and NVIDIA driver.
-
Debian 12.6 Released with Plenty of Bug Fixes and Updates
The sixth update to Debian "Bookworm" is all about security mitigations and making adjustments for some "serious problems."
-
Canonical Offers 12-Year LTS for Open Source Docker Images
Canonical is expanding its LTS offering to reach beyond the DEB packages with a new distro-less Docker image.
-
Plasma Desktop 6.1 Released with Several Enhancements
If you're a fan of Plasma Desktop, you should be excited about this new point release.