Quoting Eric W. Biederman (ebiederm@xmission.com):

"Serge E. Hallyn" [off-list ref] writes:

quoted

A common way for daemons to run with minimal privilege is to start as root,
perhaps setuid-root, choose a desired capability set, set PR_SET_KEEPCAPS,
then change uid to non-root.  A simpler way to achieve this is to set file
capabilities on a not-setuid-root binary.  However, when installing a package
inside a (user-namespaced) container, packages cannot be installed with file
capabilities.  For this reason, containers must install ping setuid-root.

Don't ping sockets avoid that specific problem?

I expect the general case still holds.

quoted

To achieve this, we would need for containers to be able to request file
capabilities be added to a file without causing these to be honored in the
initial user namespace.

To this end, the patch below introduces a new capability xattr format.  The
main enhancement over the existing security.capability xattr is that we
tag capability sets with a uid - the uid of the root user in the namespace
where the capabilities are set.  The capabilities will be ignored in any
other namespace.  The special case of uid == -1 (which must only ever be
able to be set by kuid 0) means use the capabilities in all
namespaces.

really since security.capability xattrs are currently honored in all
namespaces this isn't really necessary.  Until and unless Seth's set
changes that.

A quick comment on this.

We currently allow capabilities that have been gained to be valid in all
descendent user namespaces.

Applying this principle to the on-disk capabilities would make it so
that uid 0 would mean capabilities in all namespaces.

It might be worth it to introduce a fixed sized array with a length
parameter of perhaps 32 entries which is a path of root uids as seen by
the initial user namespace.  That way the entire construction of the
user namespace could be verified.  AKA verify the current user namespace
and the parent and the parents parent.  Up to the user namespace the
current filesystem is mounted in. We would look at how much space
allows an xattr to be stored without causing filesystems a challenge
to properly size such an array.

Given that uids are fundamentally flat that might not be particularly
useful.  If we add an alternative way of identifying user namespaces
say a privileged operation that set a uuid, then the complete path would
be more interesting.

quoted

An alternative format would use a pair of uids to indicate a range of rootids.
This would allow root in a user namespace with uids 100000-165536 mapped to
set the xattr once on a file, then launch nested containers wherein the file
could be used with privilege.  That's not what this patch does, but would be
a trivial change if people think it would be worthwhile.

This patch does not actually address the real problem, which is setting the
xattrs from inside containers.  For that, I think the best solution is to
add a pair of new system calls, setfcap and getfcap. Userspace would for
instance call fsetfcap(fd, cap_user_header_t, cap_user_data_t), to which
the kernel would, if not in init_user_ns, react by writing an appropriate
security.nscapability xattr.

That feels hard to maintain, but you may be correct that we have a small
enough userspace that it would not be a problem.

Eric

quoted

The libcap2 library's cap_set_file/cap_get_file could be switched over
transparently to use this to hide its use from all callers.

Comments appreciated.

Note - In this patch, file capabilities only work for containers which have
a root uid defined.  We may want to allow -1 uids to work in all
namespaces.  There certainly would be uses for this, but I'm a bit unsettled
about the implications of allowing a program privilege in a container where
there is no uid with privilege.  This needs more thought.

So for actually enabling (user-namespaced) containers to use these, there
are a few possibilities that come to mine.

1. A new setfcap (/getfcap) syscall.  Uses mapped uid 0 from
current_user_ns() to write a value in the security.nscapability xattr.
Userspace doesn't need to worry at all about namespace issues.

2. Just expect userspace to write a xattr;  kernel checks that no values
are changed for any other namespaces.  This could be a lot of parsing and
verifying in the kernel.

3. Switch the xattr scheme - instead of one security.nscapability xattr
with multiple entries, use security.nscapability.$(rootid).  Now the
kernel only needs to verify that the $rootid is valid for the writing
task, and we don't need a new syscall.  OTOH userspace needs to know
what it's doing.  Of course we can still hide that behind libcap2's helpers.

Any opinions on which way seems best?  1 does seem cleanest (and supports
use of seccomp if we want to forbit its use by some containers), but
involves a new pair of syscalls.  2 seems to me to be right out, but
others might disagree...

-serge