selinux: add support for RTM_NEWCHAIN, RTM_DELCHAIN, and RTM_GETCHAIN

Commit 32a4f5ecd738 ("net: sched: introduce chain object to uapi")
added new RTM_* definitions without properly updating SELinux, this
patch adds the necessary SELinux support.

While there was a BUILD_BUG_ON() in the SELinux code to protect from
exactly this case, it was bypassed in the broken commit.  In order to
hopefully prevent this from happening in the future, add additional
comments which provide some instructions on how to resolve the
BUILD_BUG_ON() failures.

Fixes: 32a4f5ecd738 ("net: sched: introduce chain object to uapi")
Cc: <stable@vger.kernel.org> # 4.19
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/nlmsgtab.c

@@ -80,6 +80,9 @@ static const struct nlmsg_perm nlmsg_route_perms[] =
 	{ RTM_NEWSTATS,		NETLINK_ROUTE_SOCKET__NLMSG_READ },
 	{ RTM_GETSTATS,		NETLINK_ROUTE_SOCKET__NLMSG_READ  },
 	{ RTM_NEWCACHEREPORT,	NETLINK_ROUTE_SOCKET__NLMSG_READ },
+	{ RTM_NEWCHAIN,		NETLINK_ROUTE_SOCKET__NLMSG_WRITE },
+	{ RTM_DELCHAIN,		NETLINK_ROUTE_SOCKET__NLMSG_WRITE },
+	{ RTM_GETCHAIN,		NETLINK_ROUTE_SOCKET__NLMSG_READ  },
 };
 
 static const struct nlmsg_perm nlmsg_tcpdiag_perms[] =
@@ -158,7 +161,11 @@ int selinux_nlmsg_lookup(u16 sclass, u16 nlmsg_type, u32 *perm)
 
 	switch (sclass) {
 	case SECCLASS_NETLINK_ROUTE_SOCKET:
-		/* RTM_MAX always point to RTM_SETxxxx, ie RTM_NEWxxx + 3 */
+		/* RTM_MAX always points to RTM_SETxxxx, ie RTM_NEWxxx + 3.
+		 * If the BUILD_BUG_ON() below fails you must update the
+		 * structures at the top of this file with the new mappings
+		 * before updating the BUILD_BUG_ON() macro!
+		 */
 		BUILD_BUG_ON(RTM_MAX != (RTM_NEWCHAIN + 3));
 		err = nlmsg_perm(nlmsg_type, perm, nlmsg_route_perms,
 				 sizeof(nlmsg_route_perms));
@@ -170,6 +177,10 @@ int selinux_nlmsg_lookup(u16 sclass, u16 nlmsg_type, u32 *perm)
 		break;
 
 	case SECCLASS_NETLINK_XFRM_SOCKET:
+		/* If the BUILD_BUG_ON() below fails you must update the
+		 * structures at the top of this file with the new mappings
+		 * before updating the BUILD_BUG_ON() macro!
+		 */
 		BUILD_BUG_ON(XFRM_MSG_MAX != XFRM_MSG_MAPPING);
 		err = nlmsg_perm(nlmsg_type, perm, nlmsg_xfrm_perms,
 				 sizeof(nlmsg_xfrm_perms));