userns: Replace netlink uses of cap_raised with capable.

userns: Replace netlink uses of cap_raised with capable.

In 2009 Philip Reiser notied that a few users of netlink connector
interface needed a capability check and added the idiom
cap_raised(nsp->eff_cap, CAP_SYS_ADMIN) to a few of them, on the premise
that netlink was asynchronous.

In 2011 Patrick McHardy noticed we were being silly because netlink is
synchronous and removed eff_cap from the netlink_skb_params and changed
the idiom to cap_raised(current_cap(), CAP_SYS_ADMIN).

Looking at those spots with a fresh eye we should be calling
capable(CAP_SYS_ADMIN).  The only reason I can see for not calling
capable is that it once appeared we were not in the same task as the
caller which would have made calling capable() impossible.

In the initial user_namespace the only difference between  between
cap_raised(current_cap(), CAP_SYS_ADMIN) and capable(CAP_SYS_ADMIN)
are a few sanity checks and the fact that capable(CAP_SYS_ADMIN)
sets PF_SUPERPRIV if we use the capability.

Since we are going to be using root privilege setting PF_SUPERPRIV
seems the right thing to do.

The motivation for this that patch is that in a child user namespace
cap_raised(current_cap(),...) tests your capabilities with respect to
that child user namespace not capabilities in the initial user namespace
and thus will allow processes that should be unprivielged to use the
kernel services that are only protected with
cap_raised(current_cap(),..).

To fix possible user_namespace issues and to just clean up the code
replace cap_raised(current_cap(), CAP_SYS_ADMIN) with
capable(CAP_SYS_ADMIN).

Acked-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>
Acked-by: Andrew G. Morgan <morgan@kernel.org>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Serge E. Hallyn <serge.hallyn@canonical.com>
Cc: Vasiliy Kulikov <segoon@openwall.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>