Re: [PATCH review 0/11] General unprivileged mount support

On Wed, Jul 06, 2016 at 10:54:40AM +0200, Jan Kara wrote:

On Mon 04-07-16 11:27:46, Eric W. Biederman wrote:

quoted

Jan Kara [off-list ref] writes:

quoted

On Sat 02-07-16 12:18:08, Eric W. Biederman wrote:

quoted

As well as in these patches the code is also available from:
git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git for-testing

It has been a long time in coming but recently in the userns tree the
superblock has been expanded with a s_user_ns field indicating the user
namespace that owns a superblock.

The s_user_ns owner of a superblock has three implications.
- Only kuids and kgids that map into s_user_ns are allowed to be sent to a
 filesystem from the vfs.
- If the uid or gid on the filesystem does not map into s_user_ns i_uid
  is set to INVALID_UID and i_gid is set to INVALID_GID.
- The scope of permission checks can be changed from global to a
  capabilitiy check in s_user_ns.

OK, to check that I understand it right:

So the uids and gids that are stored on disk are still expected to be in
the initial id namespace, aren't they?

No.

The general expectation is that the ids on disk are store in s_user_ns.

Id's that don't map to the initial id namespace get stored in the
generic data structures as INVALID_UID and INVALID_GID.

quoted

In practice I don't expect anyone will set up a situation knowingly
where id's don't map, but the case has to be handled because mistakes
and malicious code happens.

OK, thanks for explanation. But then the namespace the filesystem is
mounted with essentially becomes part of the on-disk format, doesn't it?
Because if someone mounts the media from a different namespace, suddently
the UID/GIDs may map to different users in initial user namespace and
consequences may be weird, right? Shouldn't it thus be somehow stored
together with the filesystem to make things more robust?

I don't remember the indented uses for user-ns mounts so I may be just
wrong. But my experience tells me that external data (such as user
namespace ID mappings in your case) that modify meaning of on-disk format
tend to cause maintenance difficulties in the long run... Because someone
*will* have the idea of migrating these fs images between containers /
machines and then they have to make sure mappings get migrated as well and
it all becomes cumbersome.

The intended use case for this is containers, with the idea being that I
as a user will get the same behavior in the container as I would in
init_user_ns without needing any userspace modifications to achieve
that.

So if I have a filesystem that contains uid 0 and I mount it in my
container, I should see uid 0. If I mount the same bits in another
container with a different uid mapping I should also see uid 0.

If I mkfs a new filesystem in my container then mount it, the root
directory of the fs is owned by uid 0 in my container without any
modifications to mkfs.

I'd argue that this makes it easier to migrate a disk between containers
because the ids in the disk show up the same within the container
regardless of the id mapping. If someone wants to mount a filesystem in
one container and also access it in another container with a completely
different id mapping, well I don't think that's ever going to work well.

Seth