[Devel] Re: [patch 05/10] add "permit user mounts in new namespace" clone flag

[Eric W. Biederman]

Miklos Szeredi <miklos at 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?

Eric