sysctl: allow override of /proc/sys/net with CAP_NET_ADMIN

Extend the permission check for networking sysctl's to allow modification
when current process has CAP_NET_ADMIN capability and is not root.  This
version uses the until now unused permissions hook to override the mode
value for /proc/sys/net if accessed by a user with capabilities.

Found while working with Quagga.  It is impossible to turn forwarding
on/off through the command interface because Quagga uses secure coding
practice of dropping privledges during initialization and only raising via
capabilities when necessary.  Since the dameon has reset real/effective
uid after initialization, all attempts to access /proc/sys/net variables
will fail.
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrew Morgan <morgan@kernel.org>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/sysctl_net.c

@@ -35,8 +35,22 @@ net_ctl_header_lookup(struct ctl_table_root *root, struct nsproxy *namespaces)
 	return &namespaces->net_ns->sysctl_table_headers;
 }
 
+/* Return standard mode bits for table entry. */
+static int net_ctl_permissions(struct ctl_table_root *root,
+			       struct nsproxy *nsproxy,
+			       struct ctl_table *table)
+{
+	/* Allow network administrator to have same access as root. */
+	if (capable(CAP_NET_ADMIN)) {
+		int mode = (table->mode >> 6) & 7;
+		return (mode << 6) | (mode << 3) | mode;
+	}
+	return table->mode;
+}
+
 static struct ctl_table_root net_sysctl_root = {
 	.lookup = net_ctl_header_lookup,
+	.permissions = net_ctl_permissions,
 };
 
 static LIST_HEAD(net_sysctl_ro_tables);