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
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