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

Eric W. Biederman
2016-03-06T22:07:49Z

Seth Forshee <seth.forshee@canonical.com> writes:

> On Fri, Mar 04, 2016 at 04:43:06PM -0600, Eric W. Biederman wrote:
>> Seth Forshee <seth.forshee@canonical.com> writes:
>>
>> > 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?
>>
>> 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.
>
>> 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