net: relax SO_TXTIME CAP_NET_ADMIN check

Now sch_fq has horizon feature, we want to allow QUIC/UDP applications
to use EDT model so that pacing can be offloaded to the kernel (sch_fq)
or the NIC.

Signed-off-by: Eric Dumazet <[e-mail]>
Cc: Willem de Bruijn <[e-mail]>
Acked-by: Willem de Bruijn <[e-mail]>
Acked-by: Vinicius Costa Gomes <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/sock.c

···
 		break;
 
 	case SO_TXTIME:
-		if (!ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN)) {
-			ret = -EPERM;
-		} else if (optlen != sizeof(struct sock_txtime)) {
+		if (optlen != sizeof(struct sock_txtime)) {
 			ret = -EINVAL;
+			break;
 		} else if (copy_from_user(&sk_txtime, optval,
 			   sizeof(struct sock_txtime))) {
 			ret = -EFAULT;
+			break;
 		} else if (sk_txtime.flags & ~SOF_TXTIME_FLAGS_MASK) {
 			ret = -EINVAL;
-		} else {
-			sock_valbool_flag(sk, SOCK_TXTIME, true);
-			sk->sk_clockid = sk_txtime.clockid;
-			sk->sk_txtime_deadline_mode =
-				!!(sk_txtime.flags & SOF_TXTIME_DEADLINE_MODE);
-			sk->sk_txtime_report_errors =
-				!!(sk_txtime.flags & SOF_TXTIME_REPORT_ERRORS);
+			break;
 		}
+		/* CLOCK_MONOTONIC is only used by sch_fq, and this packet
+		 * scheduler has enough safe guards.
+		 */
+		if (sk_txtime.clockid != CLOCK_MONOTONIC &&
+		    !ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN)) {
+			ret = -EPERM;
+			break;
+		}
+		sock_valbool_flag(sk, SOCK_TXTIME, true);
+		sk->sk_clockid = sk_txtime.clockid;
+		sk->sk_txtime_deadline_mode =
+			!!(sk_txtime.flags & SOF_TXTIME_DEADLINE_MODE);
+		sk->sk_txtime_report_errors =
+			!!(sk_txtime.flags & SOF_TXTIME_REPORT_ERRORS);
 		break;
 
 	case SO_BINDTOIFINDEX: