[PATCH v3 00/25] user_namespace: introduce fsid mappings

[James Bottomley]

On Tue, 2020-02-18 at 15:33 +0100, Christian Brauner wrote:
> In the usual case of running an unprivileged container we will have
> setup an id mapping, e.g. 0 100000 100000. The on-disk mapping will
> correspond to this id mapping, i.e. all files which we want to appear
> as 0:0 inside the user namespace will be chowned to 100000:100000 on
> the host. This works, because whenever the kernel needs to do a
> filesystem access it will lookup the corresponding uid and gid in the
> idmapping tables of the container. Now think about the case where we
> want to have an id mapping of 0 100000 100000 but an on-disk mapping
> of 0 300000 100000 which is needed to e.g. share a single on-disk
> mapping with multiple containers that all have different id mappings.
> This will be problematic. Whenever a filesystem access is requested,
> the kernel will now try to lookup a mapping for 300000 in the id
> mapping tables of the user namespace but since there is none the
> files will appear to be owned by the overflow id, i.e. usually
> 65534:65534 or nobody:nogroup.
> 
> With fsid mappings we can solve this by writing an id mapping of 0
> 100000 100000 and an fsid mapping of 0 300000 100000. On filesystem
> access the kernel will now lookup the mapping for 300000 in the fsid
> mapping tables of the user namespace. And since such a mapping
> exists, the corresponding files will have correct ownership.

So I did compile this up in order to run the shiftfs tests over it to
see how it coped with the various corner cases.  However, what I find
is it simply fails the fsid reverse mapping in the setup.  Trying to
use a simple uid of 0 100000 1000 and a fsid of 100000 0 1000 fails the
entry setuid(0) call because of this code:

long __sys_setuid(uid_t uid)
{
	struct user_namespace *ns =
current_user_ns();
	const struct cred *old;
	struct cred *new;
	int
retval;
	kuid_t kuid;
	kuid_t kfsuid;

	kuid = make_kuid(ns, uid);
	if
(!uid_valid(kuid))
		return -EINVAL;

	kfsuid = make_kfsuid(ns, uid);
	if
(!uid_valid(kfsuid))
		return -EINVAL;

which means you can't have a fsid mapping that doesn't have the same
domain as the uid mapping, meaning a reverse mapping isn't possible
because the range and domain have to be inverse and disjoint.

James