There is no way to limit the capabilities of usermodehelpers. This problem
reared its head recently when someone complained that any user with
cap_net_admin was able to load arbitrary kernel modules, even though the user
didn't have cap_sys_module.  The reason is because the actual load is done by
a usermode helper and those always have the full cap set.  This patch addes new
sysctls which allow us to bound the permissions of usermode helpers.

/proc/sys/kernel/usermodehelper/bset
/proc/sys/kernel/usermodehelper/inheritable

You must have CAP_SYS_MODULE  and CAP_SETPCAP to change these (changes are
&= ONLY).  When the kernel launches a usermodehelper it will do so with these
as the bset and pI.

-v2:	make globals static
	create spinlock to protect globals

-v3:	require both CAP_SETPCAP and CAP_SYS_MODULE
-v4:	fix the typo s/CAP_SET_PCAP/CAP_SETPCAP/ because I didn't commit
Signed-off-by: Eric Paris <eparis@redhat.com>
No-objection-from: Serge E. Hallyn <serge.hallyn@canonical.com>
Acked-by: David Howells <dhowells@redhat.com>
Acked-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Acked-by: Andrew G. Morgan <morgan@kernel.org>
Signed-off-by: James Morris <jmorris@namei.org>