[PATCH v2] xattr: Enable security.capability in user namespaces

[Serge E. Hallyn]

Quoting Stefan Berger (stefanb at linux.vnet.ibm.com):
> On 07/13/2017 08:38 PM, Eric W. Biederman wrote:
> >Stefan Berger <stefanb at linux.vnet.ibm.com> writes:
> >
> >>On 07/13/2017 01:49 PM, Eric W. Biederman wrote:
> >>
> >>>My big question right now is can you implement Ted's suggested
> >>>restriction.  Only one security.foo or secuirty.foo at ... attribute ?
> >>We need to raw-list the xattrs and do the check before writing them. I am fairly sure this can be done.
> >>
> >>So now you want to allow security.foo and one security.foo at uid=<> or just a single one security.foo(@[[:print:]]*)?
> >>
> >The latter.
> 
> That case would prevent a container user from overriding the xattr
> on the host. Is that what we want? For limiting the number of xattrs

Not really.  If the file is owned by a uid mapped into the container,
then the container root can chown the file which will clear the file
capability, after which he can set a new one.  If the file is not
owned by a uid mapped into the container, then container root could
not set a filecap anyway.

-serge
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