mptcp: forbit mcast-related sockopt on MPTCP sockets

Unrolling mcast state at msk dismantel time is bug prone, as
syzkaller reported:

======================================================
WARNING: possible circular locking dependency detected
5.11.0-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor905/8822 is trying to acquire lock:
ffffffff8d678fe8 (rtnl_mutex){+.+.}-{3:3}, at: ipv6_sock_mc_close+0xd7/0x110 net/ipv6/mcast.c:323

but task is already holding lock:
ffff888024390120 (sk_lock-AF_INET6){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1600 [inline]
ffff888024390120 (sk_lock-AF_INET6){+.+.}-{0:0}, at: mptcp6_release+0x57/0x130 net/mptcp/protocol.c:3507

which lock already depends on the new lock.

Instead we can simply forbit any mcast-related setsockopt

Fixes: 717e79c8 ("mptcp: Add setsockopt()/getsockopt() socket operations")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -2878,6 +2878,48 @@ static int mptcp_setsockopt_v6(struct mptcp_sock *msk, int optname,
 	return ret;
 }
 
+static bool mptcp_unsupported(int level, int optname)
+{
+	if (level == SOL_IP) {
+		switch (optname) {
+		case IP_ADD_MEMBERSHIP:
+		case IP_ADD_SOURCE_MEMBERSHIP:
+		case IP_DROP_MEMBERSHIP:
+		case IP_DROP_SOURCE_MEMBERSHIP:
+		case IP_BLOCK_SOURCE:
+		case IP_UNBLOCK_SOURCE:
+		case MCAST_JOIN_GROUP:
+		case MCAST_LEAVE_GROUP:
+		case MCAST_JOIN_SOURCE_GROUP:
+		case MCAST_LEAVE_SOURCE_GROUP:
+		case MCAST_BLOCK_SOURCE:
+		case MCAST_UNBLOCK_SOURCE:
+		case MCAST_MSFILTER:
+			return true;
+		}
+		return false;
+	}
+	if (level == SOL_IPV6) {
+		switch (optname) {
+		case IPV6_ADDRFORM:
+		case IPV6_ADD_MEMBERSHIP:
+		case IPV6_DROP_MEMBERSHIP:
+		case IPV6_JOIN_ANYCAST:
+		case IPV6_LEAVE_ANYCAST:
+		case MCAST_JOIN_GROUP:
+		case MCAST_LEAVE_GROUP:
+		case MCAST_JOIN_SOURCE_GROUP:
+		case MCAST_LEAVE_SOURCE_GROUP:
+		case MCAST_BLOCK_SOURCE:
+		case MCAST_UNBLOCK_SOURCE:
+		case MCAST_MSFILTER:
+			return true;
+		}
+		return false;
+	}
+	return false;
+}
+
 static int mptcp_setsockopt(struct sock *sk, int level, int optname,
 			    sockptr_t optval, unsigned int optlen)
 {
@@ -2886,6 +2928,9 @@ static int mptcp_setsockopt(struct sock *sk, int level, int optname,
 
 	pr_debug("msk=%p", msk);
 
+	if (mptcp_unsupported(level, optname))
+		return -ENOPROTOOPT;
+
 	if (level == SOL_SOCKET)
 		return mptcp_setsockopt_sol_socket(msk, optname, optval, optlen);