Seth Forshee [off-list ref] writes:

On Fri, Mar 04, 2016 at 04:43:06PM -0600, Eric W. Biederman wrote:

quoted

Seth Forshee [off-list ref] writes:

quoted

On Mon, Jan 04, 2016 at 12:03:50PM -0600, Seth Forshee wrote:

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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?

In general this is only an issue if uids and gids on the filesystem
do not map into the user namespace.

Yes, both capable_wrt_inode_uidgid and inode_owner_or_capable will
return true for a privileged user in the current namespace if the ids
map into that namespace.

quoted

Therefore the general fix is to limit the logic of checking for
capabilities in s_user_ns if we are dealing with INVALID_UID and
INVALID_GID.  For proc and kernfs that should never be the case
so the problem becomes a non-issue.

Further I would look at limiting that relaxation to just
inode_change_ok.  So that we can easily wrap that check per filesystem
and deny the relaxation for proc and kernfs.  proc and kernfs already
have wrappers for .setattr so denying changes when !uid_vaid and
!gid_valid would be a trivial addition, and ensure calamity does
not ensure.

Furthmore by limiting any additional to inode_change_ok we keep
the work of the additional tests off of the fast paths.

So then the inode would need to be chowned before a privileged user in a
non-init namespace would be capable towards it. That seems workable. It
looks like INVALID_UID and INVALID_GID do map into init_user_ns (which
seems a bit odd) so real root remains capable towards those indoes.

That seems okay to me then.

If I was not clear I was suggesting that we allow a sufficiently
privileged user in the filesysteme's s_user_ns to allow chowning files
with INVALID_UID and INVALID_GID.

The global root user would always be able to do that because unless
capabilities are dropped it is sufficiently privileged in ever user
namespace.

Eric