include/uapi/linux/xfrm.h: Fix XFRM_MSG_MAPPING ABI breakage

Commit 2d151d39 ("xfrm: Add possibility to set the default to block
if we have no policy") broke ABI by changing the value of the XFRM_MSG_MAPPING
enum item, thus also evading the build-time check
in security/selinux/nlmsgtab.c:selinux_nlmsg_lookup for presence of proper
security permission checks in nlmsg_xfrm_perms.  Fix it by placing
XFRM_MSG_SETDEFAULT/XFRM_MSG_GETDEFAULT to the end of the enum, right before
__XFRM_MSG_MAX, and updating the nlmsg_xfrm_perms accordingly.

Fixes: 2d151d39 ("xfrm: Add possibility to set the default to block if we have no policy")
References: https://lore.kernel.org/netdev/20210901151402.GA2557@altlinux.org/Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Acked-by: Antony Antony <antony.antony@secunet.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/uapi/linux/xfrm.h

@@ -213,13 +213,13 @@ enum {
 	XFRM_MSG_GETSPDINFO,
 #define XFRM_MSG_GETSPDINFO XFRM_MSG_GETSPDINFO
 
+	XFRM_MSG_MAPPING,
+#define XFRM_MSG_MAPPING XFRM_MSG_MAPPING
+
 	XFRM_MSG_SETDEFAULT,
 #define XFRM_MSG_SETDEFAULT XFRM_MSG_SETDEFAULT
 	XFRM_MSG_GETDEFAULT,
 #define XFRM_MSG_GETDEFAULT XFRM_MSG_GETDEFAULT
-
-	XFRM_MSG_MAPPING,
-#define XFRM_MSG_MAPPING XFRM_MSG_MAPPING
 	__XFRM_MSG_MAX
 };
 #define XFRM_MSG_MAX (__XFRM_MSG_MAX - 1)

security/selinux/nlmsgtab.c

@@ -126,6 +126,8 @@ static const struct nlmsg_perm nlmsg_xfrm_perms[] =
 	{ XFRM_MSG_NEWSPDINFO,	NETLINK_XFRM_SOCKET__NLMSG_WRITE },
 	{ XFRM_MSG_GETSPDINFO,	NETLINK_XFRM_SOCKET__NLMSG_READ  },
 	{ XFRM_MSG_MAPPING,	NETLINK_XFRM_SOCKET__NLMSG_READ  },
+	{ XFRM_MSG_SETDEFAULT,	NETLINK_XFRM_SOCKET__NLMSG_WRITE },
+	{ XFRM_MSG_GETDEFAULT,	NETLINK_XFRM_SOCKET__NLMSG_READ  },
 };
 
 static const struct nlmsg_perm nlmsg_audit_perms[] =
@@ -189,7 +191,7 @@ int selinux_nlmsg_lookup(u16 sclass, u16 nlmsg_type, u32 *perm)
 		 * 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);
+		BUILD_BUG_ON(XFRM_MSG_MAX != XFRM_MSG_GETDEFAULT);
 		err = nlmsg_perm(nlmsg_type, perm, nlmsg_xfrm_perms,
 				 sizeof(nlmsg_xfrm_perms));
 		break;