net: tls: explicitly disallow disconnect

syzbot discovered that it can disconnect a TLS socket and then
run into all sort of unexpected corner cases. I have a vague
recollection of Eric pointing this out to us a long time ago.
Supporting disconnect is really hard, for one thing if offload
is enabled we'd need to wait for all packets to be _acked_.
Disconnect is not commonly used, disallow it.

The immediate problem syzbot run into is the warning in the strp,
but that's just the easiest bug to trigger:

  WARNING: CPU: 0 PID: 5834 at net/tls/tls_strp.c:486 tls_strp_msg_load+0x72e/0xa80 net/tls/tls_strp.c:486
  RIP: 0010:tls_strp_msg_load+0x72e/0xa80 net/tls/tls_strp.c:486
  Call Trace:
   <TASK>
   tls_rx_rec_wait+0x280/0xa60 net/tls/tls_sw.c:1363
   tls_sw_recvmsg+0x85c/0x1c30 net/tls/tls_sw.c:2043
   inet6_recvmsg+0x2c9/0x730 net/ipv6/af_inet6.c:678
   sock_recvmsg_nosec net/socket.c:1023 [inline]
   sock_recvmsg+0x109/0x280 net/socket.c:1045
   __sys_recvfrom+0x202/0x380 net/socket.c:2237

Fixes: 3c4d755 ("tls: kernel TLS support")
Reported-by: syzbot+b4cd76826045a1eb93c1@syzkaller.appspotmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250404180334.3224206-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/tls/tls_main.c

@@ -852,6 +852,11 @@ static int tls_setsockopt(struct sock *sk, int level, int optname,
 	return do_tls_setsockopt(sk, optname, optval, optlen);
 }
 
+static int tls_disconnect(struct sock *sk, int flags)
+{
+	return -EOPNOTSUPP;
+}
+
 struct tls_context *tls_ctx_create(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
@@ -947,6 +952,7 @@ static void build_protos(struct proto prot[TLS_NUM_CONFIG][TLS_NUM_CONFIG],
 	prot[TLS_BASE][TLS_BASE] = *base;
 	prot[TLS_BASE][TLS_BASE].setsockopt	= tls_setsockopt;
 	prot[TLS_BASE][TLS_BASE].getsockopt	= tls_getsockopt;
+	prot[TLS_BASE][TLS_BASE].disconnect	= tls_disconnect;
 	prot[TLS_BASE][TLS_BASE].close		= tls_sk_proto_close;
 
 	prot[TLS_SW][TLS_BASE] = prot[TLS_BASE][TLS_BASE];