On Fri, Jun 23, 2017 at 10:01:46AM +0300, Amir Goldstein wrote:

On Thu, Jun 22, 2017 at 9:59 PM, Stefan Berger
[off-list ref] wrote:

quoted

This series of patches primary goal is to enable file capabilities
in user namespaces without affecting the file capabilities that are
effective on the host. This is to prevent that any unprivileged user
on the host maps his own uid to root in a private namespace, writes
the xattr, and executes the file with privilege on the host.

We achieve this goal by writing extended attributes with a different
name when a user namespace is used. If for example the root user
in a user namespace writes the security.capability xattr, the name
of the xattr that is actually written is encoded as
security.capability at uid=1000 for root mapped to uid 1000 on the host.
When listing the xattrs on the host, the existing security.capability
as well as the security.capability at uid=1000 will be shown. Inside the
namespace only 'security.capability', with the value of
security.capability at uid=1000, is visible.

Am I the only one who thinks that suffix is perhaps not the best grammar
to use for this namespace?
xattrs are clearly namespaced by prefix, so it seems right to me to keep
it that way - define a new special xattr namespace "ns" and only if that
prefix exists, the @uid suffix will be parsed.
This could be either  ns.security.capability at uid=1000 or
ns at uid=1000.security.capability. The latter seems more correct to me,
because then we will be able to namespace any xattr without having to
protect from "unprivileged xattr injection", i.e.:
setfattr -n "user.whatever.foo at uid=0"

Amir.

Hi Amir,

I was liking the prefix at first, but I'm actually not sure it's worth
it.  THe main advantage would be so that checking for namespace or other
tags could be done always at the same offset simplifying the parser.
But since we will want to only handle namespacing for some tags, and
potentially differently for each task, it won't actually be simpler, I
don't think.

On the other hand we do want to make sure that the syntax we use is
generally usable, so I think simply specifying that >1 tags can each
be separate by '@' should suffice.  So for now we'd only have

	security.capability at uid=100000

soon we'd hopefully have

	security.ima at uid=100000

and eventually trusted.blarb at foo=bar

-serge
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