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When Not to Use chroot
Hyena writes "Linux guru Alan Cox is quoted as saying 'chroot is not and never has been a security tool' in a KernelTrap article summarizing a lengthy thread on the Linux Kernel mailing list. The discussion began with a patch attempting to 'fix a security hole' in the Unix chroot command, trying to improve the ability of chroot to contain a process. When it was pointed out that people have been using chroot as a security tool for years, another kernel hacker retorted, 'incompetent people implementing security solutions are a real problem.' A quick search on the terms 'chroot+security' quickly reveals that many people have long thought (wrongly) that chroot's purpose was for improving security."
And RAID isn't for safety of your data either, hey?
Locks on your house aren't for security, they're just to keep the door closed if a cat pushes on it, right?
Seatbelts aren't to prevent you from flying through a windshield, they're just there so you don't slide around while taking corners.
Sorry, chroot *is* a security tool; it's very much useful for security. Maybe it wasn't written as one - maybe it was never intended to be one, but it *is* one now, no matter what Alan Cox says.
Software, especially open source software, is a lot like language. Despite the best efforts of nitpicking English teachers everywhere, the meaning of both words and code are whatever the vast majority agrees upon. And regardless of that, you may call me crazy, but the ability to restrict what a user can and can't access; what a process can or can't access, sounds like a security tool to me.
The purpose of chroot is to change the root directory. Chroot is particularly useful for recovery and diagnostics.
If you system that won't boot due to a boot sector problem Boot from a CD, mount your partitions, chroot to your root partition and run lilo/grub/... to rewrite your boot sector.
If you system that won't boot due to init script problems Boot from a CD, mount your partitions, chroot to your root partition and run run your full init process. If you run into problems, rerun your init scripts rather than rebooting.
Unfortunately, many people think chroot is a security tool so many people don't think it in non-security contexts.
Do all OS developers become assholes? I've done a lot with VxWorks and I hope I don't become as twisted as these folk. I better just stay away from authoring my own kernel.
Honestly, I might be in the classification of people who don't understand, but I resent the implication of "incompetent". I really hate the idea that you have to be an all-knowledgeable ubergeek, or else stay completely away from computers.
It seems like a simple issue: people have obviously felt the need to jail users for security reasons. They've been lead by someone to believe that chroot is a solution. If chroot isn't the solution, then why not give people a better solution instead of calling them incompetent.
It reminds me of a discussion that I was involved in a while back. I'll tell the story:
So all I wanted was to know how to do something, and everyone thought it was a lot of fun to tell me how incompetent I was. If the answer is so obvious, why not explain it? More to the point, if you're such a fricken genius, why not figure out a way to get people the functionality they want in a form they'll understand? I still don't understand why secure authentication is a silly thing to want.
Assuming that everyone running a server is going to be a super-genius who wants to spend all day researching everything-- having that expectation is retarded. I've been working in IT for a while, and I'll tell you right now that there are an awful lot of admins that are way dumber than I am. A solution that only super-geniuses can figure out isn't a practical solution because no one will use it.
So if a lot of people want to jail users into a specific directory for various reasons, why can't we have that functionality? If one particular method (in this case, chroot) doesn't do a good job of jailing users, then can one of the super-geniuses out there come up with a good/real/practical/secure method for accomplishing that?
If you can't, please refrain from name-calling because they want to do something that you can't figure out how to accomplish.
Please tell me that none of those bone-heads on LKVM advocating that chroot should be 'root proof' haven't had any patches accepted!
/tmp), that is free of any setuid binaries, and without "useful" utilities like wget or curl that can make exploiting the system child's play. If your program runs inside of a chroot as a non-root user, and your chroot has no setuid binaries, and your kernel has no privilege escalation vulns, then you can be reasonably sure that nobody will break the chroot or achieve privilege escalation. Without a chroot, you would have to clear your entire server of setuid binaries and mode 7771 directories -- not to mention the potential for intentionally world-readable files that can lead to information exposure. Quite simply, a chroot prevents an arbitrary-execution vulnerability in bind (or other process) from exploiting a privilege escalation vulnerability in apache (or other process).
Of course chroot() doesn't do any good if a process inside of it is running as root. This is very well known. However, that doesn't make chroot() useless, it is still plenty useful. If you execute chroot() and then a seteuid(uid) where uid>0, then you prevent a hole/bug in your program from being exploited in a way that will allow file access/execution outside the chroot. That *is* a security advantage.
The point of "chroot security", cases where chroot is used to improve security, isn't to contain a malicious root user. The point is to prevent privilege escalation. You can create a chroot without any directories with mode 7771 privileges (a la
What some people think, apparently due to pure ignorance, is that chroot() is an end-all solution that will prevent even a root-owned process from accessing files outside the chroot, or worse, thinking that it protects the memory subsytem in any way. It doesn't. Even if the discussed patch was applied to the kernel, a root-owned process could still alter kernel memory, access raw devices, etc.
Improvements in ACLs under Linux minimize some of the needs for a chroot, other than the fact that a chroot is still much easier to configure and ACLs do not handle all of the use-cases that a chroot can solve. (and visa-versa, chroot cannot solve all of the problems solved by ACLs) Additionally, a chroot *and* ACLs can be used together for further-improved security.
$ ls -l
-rwxr-xr-x 1 root root 700560 2007-04-11 09:32
$ ln
$ ls -l foo
-rwxr-xr-x 2 root root 700560 2007-04-11 09:32 foo
Uhhh, why is a regular user allowed to create a file owned by root? Apparently, you don't know what a hard link is.
You haven't created a file owned by root. You've created an i-node pointing to the data blocks of a file owned by root.
If root were to rm
Your way, I could do the following on a file with 600 permission:
cd
ln
chmod 666 mine
cat mine
Nice and easy way to get around a 600 permission.
The behavior is correct, not a bug.
Regards,
--Keith
Man, things like this make me want to NOT switch to Linux... Even though I had a better experience with Ubuntu that I did Vista. What's your problem with that statement?
It's absolutely true and it is not limited to linux.
Let's take it a few more steps further as an example: 'incompetent people designing bridges are a real problem.'
'incompetent people performing surgery are a real problem.'
'incompetent people running the government are a real problem.'
Do you have a problem with any of those statements?
If you don't even know what chroot() is, then you are not the target of the man's complaint.
When information is power, privacy is freedom.
Go ahead. One of the (many) differences between Vista and Linux is that if you want to, you can march up to any of the core Linux kernel architects and tell them they have some fundamental long-standing unix interface totally wrong. The flip side of that is that they also won't stop anyone from flaming you if you do that.
And that's exactly what happened here. This guy wasn't posting a question on a local LUG. He was posting to the Linux kernel mailing list--the place where people actually meet to do kernel development. And he wasn't asking a question, he was arguing with people like Al Viro, a primary architect of the Linux filesystem api's. Which would be great if he was correct. But in fact he was totally wrong. And even that would be OK if he took the time to do his homework and to listen carefully when people explained the issue to him.
But he didn't really, so as a result he got a few flames. Some of the posters to lkml aren't polite in such a situation. I think that's kind of understandable, though actually agree that that's a problem. Are the core Vista kernel developers any better? Who knows? Does the general public doesn't have the option of participating in their development forums?