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Chroot in OpenSSH

Posted by ScuttleMonkey on from the making-life-easier-always-my-goal dept.

bsdphx writes "OpenSSH developers Damien Miller and Markus Friedl have recently added a nifty feature to make life easier for admins. Now you can easily lock an SSH session into a chroot directory, restrict them to a built-in sftp server and apply these settings per user. And it's dead simple to do. If you need to allow semi-trusted people on your computers, then you want this bad!"

44 of 62 comments (clear)

  1. Why bother? by whoever57 · · Score: 3, Insightful

    Didn't we just read that chroot "jails" are not secure?

    --
    The real "Libtards" are the Libertarians!
    1. Re:Why bother? by bsdphx · · Score: 5, Insightful

      Understanding the issues is better than parroting what you've heard from random sources. Given the OpenBSD and OpenSSH track record for security it's obvious they have some serious clues about security.

    2. Re:Why bother? by Wesley+Felter · · Score: 3, Interesting

      Didn't we just read that chroot "jails" are not secure? I've read those arguments and find them confusing. Sure, root can break out of a chroot, but what about non-root users?
    3. Re:Why bother? by illegibledotorg · · Score: 5, Informative

      Giving someone a shell and putting them in a chroot crafted to look and function like a full system is one thing.

      Giving someone an SFTP session and chrooting them into a subdirectory is another thing.

      The feature added in this commit was arguably intended for the latter purpose given the additional changes to the SFTP subsystem that were included. There are countless tutorials and patches and scripts that are available to achieve chrooted SFTP-only access, but now it's been implemented in the core of OpenSSH. In my eyes, this solution is not only a "cleaner" solution to the problem, but it's probably more secure too.

    4. Re:Why bother? by jandrese · · Score: 4, Informative

      They are probably better than giving semi-trusted users full filesystem access, even if they aren't perfect security. It's not even that chroot is inherently broken, it's just that people were using it incorrectly (setting it suid or letting the user become root inside of their jail). Most of the complaints seem to be "the user managed to get root and broke out of the jail", which is a problem with whatever allowed your user to become root in the first place, not the jail itself.

      Basically, to break out of a chroot you need to be root. If you're root, then you've already defeated the security on the box anyway. Don't let untrusted users become root.

      --

      I read the internet for the articles.
    5. Re:Why bother? by parcel · · Score: 4, Informative

      Didn't we just read that chroot "jails" are not secure? You may want to take a look at http://www.openbsd.org/faq/faq10.html#httpdchroot, especially the section titled "Should I use the chroot feature?".

      I imagine something similar would be forthcoming regarding OpenSSH specifically.
    6. Re:Why bother? by jjohnson · · Score: 3, Informative

      They're not secure for root users. That was the issue identified in the recent "read" you mention--someone was pointing out that root can break out in a particular way, and the kernel devs responded that 1) that was by design, and 2) locking that down left an infinite number of other ways to break out if you're root.

      For regular user accounts, a properly configured chroot jail is still a very useful security tool.

      --
      Anyone who loves or hates any language, platform, or manufacturer, doesn't know what they're talking about.
    7. Re:Why bother? by ianezz · · Score: 1
      Didn't we just read that chroot "jails" are not secure?

      Only when you have full shell access. This patch is just about confining sftp file transfers via chroot(2) for some users without the burden of setting up a full chrooted environment. Sounds really sweet.

    8. Re:Why bother? by Brian+Gordon · · Score: 1

      I don't really know anything about chroot security either, but the first thing that occurs to me is that there's way too much code running/setuid'd as root or in kernel/driver space to just change the filesystem interface and expect that to be secure. You can't possibly lock down every interface in all of this code without a more pervasive solution.. especially since we can barely (read: we can't) keep code listening on internet-accessable ports secure from buffer/integer overflows and other tricky business. How on earth are you going to lock down everything from cron to ls?

    9. Re:Why bother? by mike_sucks · · Score: 1
      "Giving someone an SFTP session and chrooting them into a subdirectory is another thing."

      Yep, that's precisely why my ears perked up upon hearing this, for hosting providers that want to provide secure remote file access, this is awesome!

      /Mike

      --
      -- "So, what's the deal with Auntie Gerschwitz et all?"
    10. Re:Why bother? by schon · · Score: 2, Informative

      that's precisely why my ears perked up upon hearing this, for hosting providers that want to provide secure remote file access, this is awesome My question is: what took so long?

      The (now defunct) commercial SSH has had this feature for almost 10 years.
    11. Re:Why bother? by mike_sucks · · Score: 1
      Yeah, good question. I dunno - maybe everyone was standing around asking "when is someone going to do this?" and using FTP(S) in the mean time. I might have been. ;)

      /mike

      --
      -- "So, what's the deal with Auntie Gerschwitz et all?"
    12. Re:Why bother? by jjohnson · · Score: 1

      You misunderstood that the whole debate was about what a root user could do from within a chroot jail. A properly limited user account can be secured in a chroot jail (i.e., not simply chrooting but configuring a jailed environment).

      The devs were correct that chroot alone was not designed for the purpose of security and can't be used alone to provide any.

      --
      Anyone who loves or hates any language, platform, or manufacturer, doesn't know what they're talking about.
  2. Bad to the bone by Anonymous Coward · · Score: 1, Funny

    I want it good. Perhaps you meant badly? Have you seen that new Ferrari? That's one bad piece of machinery!

    (However, in this case you're correct in your usage.)
    1. Re:Bad to the bone by nuzak · · Score: 1

      I love the PowerGlove, it's so Bad!

      --
      Done with slashdot, done with nerds, getting a life.
  3. Oh thank god by Just+Some+Guy · · Score: 2, Interesting

    Now I can finally switch some customers from FTP to SFTP. Thanks for making this hugely useful change!

    Anyone know if SFTP logging will be added any time soon? That's the last missing feature i always have to manually patch in.

    --
    Dewey, what part of this looks like authorities should be involved?
    1. Re:Oh thank god by pembo13 · · Score: 2, Insightful

      All we need now is some form of virtual user system that can be mapped to a real, unprivileged user, preferably with a flexible auth system.

      --
      "Thanks for all the money you paid to us. We've used it to buy off ISO among other things" -Microsoft
    2. Re:Oh thank god by David_W · · Score: 1

      That's the last missing feature i always have to manually patch in.

      I'm still hankering for tab completion in SFTP myself... maybe someday.

    3. Re:Oh thank god by PylonHead · · Score: 2, Informative

      Use lftp

      It will let you connect to sftp servers, and have a sane command line experience. It also has many nifty mirroring commands.

      --
      # (/.);;
      - : float -> float -> float =
    4. Re:Oh thank god by Just+Some+Guy · · Score: 1

      For real? My sarcasm meter is out of whack today so I can't tell.

      --
      Dewey, what part of this looks like authorities should be involved?
    5. Re:Oh thank god by pembo13 · · Score: 1

      For real

      --
      "Thanks for all the money you paid to us. We've used it to buy off ISO among other things" -Microsoft
    6. Re:Oh thank god by Just+Some+Guy · · Score: 1

      OK, just checking. It's been a long day. :-)

      Anyway, would something like Kerberos fill the bill for you? I use it to navigate around our network, and it's not that hard to do once you get over the initial learning curve. If you're stucking working with Windows, you can even auth against an Active Directory domain.

      --
      Dewey, what part of this looks like authorities should be involved?
  4. Tell us more. by Valdrax · · Score: 1

    I'd like to hear more about methods of breaking out of a chroot jail without becoming root during the process.

    --
    If it's for-profit but free, you're not the customer -- you're the product (e.g., the Slashdot Beta's "audience").
    1. Re:Tell us more. by jandrese · · Score: 2, Informative

      Yeah, every method I've seen starts with:

      Step 1: Become root

      Once you are root, there are dozens of ways to break out of a jail (all the way from modifying kernel memory structures directly to rewriting inodes to installing a kernel module that grants you access to whatever you need, etc...

      --

      I read the internet for the articles.
    2. Re:Tell us more. by sjames · · Score: 4, Informative

      In the right circumstances, 2 non-root users can conspire to break out of jail if one is chrooted below the other.

      Let's say A is chrooted to /home/sorta-trusted and B to /home/sorta-trusted/not-so-much.

      A diropens his / and creates a unix socket in /not-so-much. B opens the socket in his /. Now, A passes his fd to his / to B. B then does fdchdir on the fd and he's out of jail. Now B can break A out.

      The moral is, never use nested chroot jails!

    3. Re:Tell us more. by Just+Some+Guy · · Score: 1

      A diropens his / and creates a unix socket in /not-so-much. B opens the socket in his /. Now, A passes his fd to his / to B. B then does fdchdir on the fd and he's out of jail. Now B can break A out.

      That's pretty interesting! In practice, though, what would that get them? B would now have access to A's chroot (possibly - file permissions might be too restrictive anyway). If A trusts B that much, they could just share A's login.

      Not saying this isn't serious, but I'm too tired to think of a useful attack vector at this moment. Am I missing something?

      --
      Dewey, what part of this looks like authorities should be involved?
    4. Re:Tell us more. by sjames · · Score: 2, Informative

      It's far worse, at least in the Linux kernel (and quite probably other Unix as well but I haven't studied them). The linux kernel assumes that the PWD is at or below the chroot. When a system call parses a pathname, it substitutes the chroot for a leading /, and when walking through a .. in a pathname, it checks that the current directory in the walk isn't already the chroot before following .. up.

      So, once B gets to A's chroot (/home/sorta-trusted), it can access the real / as ../.. because now, the ..'s in the path won't pass through the stored chroot directory (/home/sorta-trusted/not-so-much. So, B diropens ../.. and passes that to A.

      So, interestingly, A can grant B access to something it doesn't have itself. B can then return the favor. Many argue that chroot isn't a security measure in the first place, so it's firmly WONT FIX.

      Personally, I say it IS a bug since right or wrong, it is used as a security measure all the time (and is quite a useful one but for the holes). I'm testing a patch that closes the escape even for root now (unless root hacks around in the kernel's memory of course but that can also be closed in a capabilities system).

    5. Re:Tell us more. by Just+Some+Guy · · Score: 1

      You know, I've had two "oh crap!" moments involving computer security. The first happened when I learned about SQL injection. The second occurred just now when I read your letter. Thanks for the followup!

      --
      Dewey, what part of this looks like authorities should be involved?
  5. Why is this news? by Abattoir · · Score: 1

    I was doing openssh(+sftp) with chroot on Solaris 2.6 several years ago. Does this have some Ubuntu GUI to make it easy or something?

    1. Re:Why is this news? by Doctor+Crumb · · Score: 2, Informative

      This is news because the chroot and sftp server are now built in to the openssh binaries, so you don't have to manually set up the chroot. While there's no GUI, it is in fact now easier to set up such a thing.

    2. Re:Why is this news? by LurkerXXX · · Score: 1

      Ubuntu GUI? Where do you get Ubuntu from? This is from the folks who make OpenSSH. And OpenSSH is made by the OpenBSD folks, not Ubuntu.

      This simply makes it much easier to do what many folks have been setting up manually for a long time. I'm hoping they'll follow it up with SCP support as well. Time to buy another CD set to support the project :)

  6. Nothing New by NYFreddie · · Score: 2, Informative

    This isn't really anything new. This functionality has existed as a patch for a while. It's still nice to see that it's finally being integrated into the main tree, though.

    --
    Barbie of Borg - She doesn't just Assimilate, She Accessorizes too!
  7. FTP servers have been doing this for years by caseih · · Score: 3, Informative

    The purpose of this feature doesn't seem to be to restrict what a shell user can do. Rather, if I read this correctly, it restricts what files a user can access via sftp. Without this feature, a user can sftp in, and then cd to / or any other folder that he has rights too. This chroot feature lets the admin limit the root to, say, his home directory, or some other folder such as a virtual web root or something.

    It's only natural that this same chroot feature would be added to sftp.

  8. Does This Mean by ajs318 · · Score: 2, Interesting

    Does this mean that I can give users shell access, by placing (hard links to) a stripped-down busybox and ash in $HOME/bin, and they won't be able to access anything outside the chroot environment? That could be sweet.

    --
    Je fume. Tu fumes. Nous fûmes!
  9. all that for sftp? by sgt+scrub · · Score: 3, Interesting

    It is cool tech but not the way I would do things. WebDav with ApacheSSL properly installed is lots safer. IMHO there should never be user accounts on a machine, other than root and the person administrating the box.

    --
    Having to work for a living is the root of all evil.
    1. Re:all that for sftp? by evilviper · · Score: 3, Insightful

      WebDav with ApacheSSL properly installed is lots safer.

      Why? No privilege separation. A MUCH bigger code base.

      Not to mention fewer standalone programs.

      IMHO there should never be user accounts on a machine,

      Why not? The user security model is reliable and time tested. It does not require reinventing the "user". It does not depend on one program handling it's own system of virtual permissions correctly. It does not depend on the security of a large program that users directly interact with.

      I can see ample reasons sftp is safer.
      --
      Slashdot gets worse every day... Pipedot: News for nerds, without the corporate slant
    2. Re:all that for sftp? by evilviper · · Score: 1

      You're welcome to break into any of my systems using a user account that has been disabled, and has no permission to access anything. eg. user "nobody". That's precisely how I would set up a sftp group.

      --
      Slashdot gets worse every day... Pipedot: News for nerds, without the corporate slant
  10. The idea by warrax_666 · · Score: 1

    is that cron and ls aren't in the chrooted filesystem. That's why they're (supposedly) more secure than just running the daemons "from" /.

    --
    HAND.
  11. Re:Oh thank god - vsftpd v sftpd vs. vs ftpd. by anon+mouse-cow-aard · · Score: 1

    I really love how vsftpd works.

    This is a "Very Secure FTP Daemon" I would love for it to be configured exactly the same, but
    the transport protocol would be Sech-file-xfer draft protocol (SFTP)

    vssftpd ?

    how about just a protocol option in the config of vsftpd...

  12. RSSH ? by ElberethZone · · Score: 1

    This looks similar to the features in RSSH http://www.pizzashack.org/rssh/...

  13. Re:IMPORTANT MESSAGE FOR bsdphx!!!!! by khellendros1984 · · Score: 1

    I think English speakers are slowly dropping use of "ly" on the ends of adjectives. If so, it's one of those things that will slowly happen more and more, until that's just the way it's done.

    --
    It is pitch black. You are likely to be eaten by a grue.
  14. Re:IMPORTANT MESSAGE FOR bsdphx!!!!! by bytesex · · Score: 1

    'If so, it's one of those things that will slow happen more and more, until that's just the way it's done.'

    Did that sound good to you ? (Ironically, the word 'good' should really have been 'well' in the previous sentence)

    --
    Religion is what happens when nature strikes and groupthink goes wrong.
  15. Re:IMPORTANT MESSAGE FOR bsdphx!!!!! by IpalindromeI · · Score: 1

    Ironically, the word 'good' should really have been 'well' in the previous sentence

    Did it change the meaning to something other than what was intended? Because that's what ironic means.

    Making the same mistake yourself while correcting someone else is not ironic. It's just humorously coincidental. Please watch this episode of Futurama for further education.

    --

    --
    Promoting critical thinking since 1994.
  16. Re:how chroot jails helps you when you got this by Just+Some+Guy · · Score: 1

    it doesn't, unless you want not to allow users to run anything?

    Since SFTP stands for Secure File Transfer Protocol, it's not really designed to allow you to execute commands.

    --
    Dewey, what part of this looks like authorities should be involved?