Messages in this thread | | | From | (Eric W. Biederman) | Subject | Re: [Devel] Re: [patch 05/10] add "permit user mounts in new namespace" clone flag | Date | Mon, 16 Apr 2007 13:16:48 -0600 |
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Miklos Szeredi <miklos@szeredi.hu> 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.
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?
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