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    SubjectRe: [Devel] Re: [patch 05/10] add "permit user mounts in new namespace" clone flag
    Quoting Eric W. Biederman (
    > Miklos Szeredi <> writes:
    > >> > That depends. Current patches check the "unprivileged submounts
    > >> > allowed under this mount" flag only on the requested mount and not on
    > >> > the propagated mounts. Do you see a problem with this?
    > >>
    > >> I think privileges of this sort should propagate. If I read what you
    > >> just said correctly if I have a private mount namespace I won't be able
    > >> to mount anything unless when it was setup the unprivileged submount
    > >> command was explicitly set.
    > >
    > > By design yes. Why is that a problem?
    > It certainly doesn't match my intuition.
    > Why are directory permissions not sufficient to allow/deny non-priveleged mounts?
    > I don't understand that contention yet.

    The same scenarios laid out previously in this thread. I.e.

    1. user hallyn does mount --bind / /home/hallyn/root
    2. (...)
    3. admin does "deluser hallyn"

    and deluser starts wiping out root


    1. user hallyn does mount --bind / /home/hallyn/root
    2. backup daemon starts backing up /home/hallyn/root/home/hallyn/root/home...

    So we started down the path of forcing users to clone a new namespace
    before doing user mounts, which is what the clone flag was about. Using
    per-mount flags also suffices as you had pointed out, which is being
    done here. But directory permissions are inadequate.

    (Unless you want to tackle each problem legacy tool one at a time to
    remove problems - i.e. deluser should umount everything under
    /home/hallyn before deleting, backup should be spawned from it's own
    namespace cloned right after boot or just back up on one filesystem,


    > I should probably go back and look and see how plan9 handles mount/unmount
    > permissions. Plan9 gets away with a lot more because it doesn't have
    > a suid bit and mount namespaces were always present, so they don't have
    > backwards compatibility problems.
    > My best guess at the moment is that plan9 treated mount/unmount as
    > completely unprivileged and used the mount namespaces to limit the
    > scope of what would be affected by a mount/unmount operation. I think
    > that may be reasonable in linux as well but it will require the
    > presence of a mount namespace to limit the affects of what a user can
    > do.
    > So short of a more thorough audit I believe the final semantics should
    > be:
    > - mount/unmount for non-priveleged processes should only be limited
    > by the mount namespace and directory permissions.
    > - CLONE_NEWNS should not be a privileged operation.
    > What prevents us from allowing these things?
    > - Unprivileged CLONE_NEWNS and unprivileged mounts needs resource
    > accounting so we don't have a denial of service attack.
    > - Unprivileged mounts must be limited to directories that we have
    > permission to modify in a way that we could get the same effect
    > as the mount or unmount operation in terms of what files are visible
    > otherwise we can mess up SUID executables.
    > - Anything else?
    > There are user space issues such as a reasonable pam module and how
    > to do backups. However those are user space issues.
    > What am I missing that requires us to add MNT_USER and MNT_USERMNT?
    > Eric
    > -
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      Last update: 2007-04-16 22:01    [W:0.025 / U:34.884 seconds]
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