Re: [PATCH RESEND v2 11/18] fs: Ensure the mounter of a filesystem is privileged towards its inodes

[Seth Forshee <seth.forshee@canonical.com>]

On Mon, Jan 04, 2016 at 12:03:50PM -0600, Seth Forshee wrote:
> The mounter of a filesystem should be privileged towards the
> inodes of that filesystem. Extend the checks in
> inode_owner_or_capable() and capable_wrt_inode_uidgid() to
> permit access by users priviliged in the user namespace of the
> inode's superblock.

Eric - I've discovered a problem related to this patch. The patches
you've already applied to your testing branch make it so that s_user_ns
can be an unprivileged user for proc and kernfs-based mounts. In some
cases DAC is the only thing protecting files in these mounts (ignoring
MAC), and with this patch an unprivileged user could bypass DAC.

There's a simple solution - always set s_user_ns to &init_user_ns for
those filesystems. I think this is the right thing to do, since the
backing store behind these filesystems are really kernel objects.  But
this would break the assumption behind your patch "userns: Simpilify
MNT_NODEV handling" and cause a regression in mounting behavior.

I've come up with several possible solutions for this conflict.

 1. Drop this patch and keep on setting s_user_ns to unprivilged users.
    This would be unfortunate because I think this patch does make sense
    for most filesystems.
 2. Restrict this patch so that a user privileged towards s_user_ns is
    only privileged towards the super blocks inodes if s_user_ns has a
    mapping for both i_uid and i_gid. This is better than (1) but still
    not ideal in my mind.
 3. Drop your patch and maintain the current MNT_NODEV behavior.
 4. Add a new s_iflags flag to indicate a super block is from an
    unprivileged mount, and use this in your patch instead of s_user_ns.

Any preference, or any other ideas?

Thanks,
Seth