Zack's Kernel News
VFS Attacks and Chroot Breakouts
Eric W. Biederman wanted to clean up some problems with the Virtual Filesystem (VFS), but he wanted to make sure his changes wouldn't break anything.
First, he pointed out that because not all filesystem changes went through the VFS, filesystems had to lie to the VFS to make everything work out OK for those kinds of changes.
Second, and perhaps more importantly, VFS at that time had a bug (or misfeature) that allowed a hostile user to mount a filesystem on another user's dentry
, such that the first user couldn't get rid of it. Bad scene.
Eric's solution, he said, would fix the problems, reduce the overall complexity of the VFS, and make the kernel source tree smaller. However, because it involved a change in the semantics of the VFS interfaces, he wanted to proceed with caution and make sure nothing would suddenly break.
Linus Torvalds said that the semantic change was actually very big. Specifically, the rmdir()
system call had previously refused to remove an empty directory whereas, after Eric's change, rmdir()
would do it without complaint. Linus pointed out that there was code in Git itself that relied on the existing behavior of rmdir()
. He said he didn't know if this would translate into anyone in the world really and truly experiencing any inconvenience, but the semantic change seemed big to him.
Linus added, "I like the _concept_ of being able to remove a mount-point and the mount just goes away. But I do think that for sanity sake, it should have something like 'if one of the mounts is in the current namespace, return -EBUSY
'. IOW, the patch-series would make the VFS layer _able_ to remove mount-points, but a normal rmdir()
when something is mounted in that namespace would fail in order to give legacy behavior."
Eric said he had no problem making that change, though he felt it "would be more about making stupid mistakes like running rm -rf /
fail than it would be about security or correctness."
Intuitively, Eric said he agreed with Linus that "it is less surprising if rmdir was forbidden in mount namespaces where it has a mount."
At this point, the conversation veered off into other "there are security holes we should fix" topics. Rob Landley said he wanted to fix the problem with chroot()
, which would allow any user to easily break out of it using a simple set of commands from within a chrooted environment:
chdir("/"); mkdir("sub"); chroot("sub"); chdir("./../../../../../../../..");
But Linus said, "escaping chroot is usually fairly easy. Making an escape-proof chroot is really quite hard. Basically impossible if you allow root." And Al Viro said, "the whole thing is idiocy – chroot()
is not and has never been root-proof and anybody expecting it to be has failed to read any number of FAQs out there."
Linus also said, "That doesn't make chroot()
useless – it just means that the uses are elsewhere (it's useful for various non-security issues like development environments, but it can also be useful as one small _part_ of some bigger model, like a VM etc)."
Linus added, "pivot_root()
does end up being a 'better chroot than chroot' if you're looking for containment. It may not be a pretty system call, but it does avoid at least the most obvious gotchas with chroot()
."
« Previous 1 2
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.
News
-
Canonical Releases Ubuntu 24.04
After a brief pause because of the XZ vulnerability, Ubuntu 24.04 is now available for install.
-
Linux Servers Targeted by Akira Ransomware
A group of bad actors who have already extorted $42 million have their sights set on the Linux platform.
-
TUXEDO Computers Unveils Linux Laptop Featuring AMD Ryzen CPU
This latest release is the first laptop to include the new CPU from Ryzen and Linux preinstalled.
-
XZ Gets the All-Clear
The back door xz vulnerability has been officially reverted for Fedora 40 and versions 38 and 39 were never affected.
-
Canonical Collaborates with Qualcomm on New Venture
This new joint effort is geared toward bringing Ubuntu and Ubuntu Core to Qualcomm-powered devices.
-
Kodi 21.0 Open-Source Entertainment Hub Released
After a year of development, the award-winning Kodi cross-platform, media center software is now available with many new additions and improvements.
-
Linux Usage Increases in Two Key Areas
If market share is your thing, you'll be happy to know that Linux is on the rise in two areas that, if they keep climbing, could have serious meaning for Linux's future.
-
Vulnerability Discovered in xz Libraries
An urgent alert for Fedora 40 has been posted and users should pay attention.
-
Canonical Bumps LTS Support to 12 years
If you're worried that your Ubuntu LTS release won't be supported long enough to last, Canonical has a surprise for you in the form of 12 years of security coverage.
-
Fedora 40 Beta Released Soon
With the official release of Fedora 40 coming in April, it's almost time to download the beta and see what's new.