rxrpc: Limit the listening backlog

Limit the socket incoming call backlog queue size so that a remote client
can't pump in sufficient new calls that the server runs out of memory.  Note
that this is partially theoretical at the moment since whilst the number of
calls is limited, the number of packets trying to set up new calls is not.
This will be addressed in a later patch.

If the caller of listen() specifies a backlog INT_MAX, then they get the
current maximum; anything else greater than max_backlog or anything
negative incurs EINVAL.

The limit on the maximum queue size can be set by:

	echo N >/proc/sys/net/rxrpc/max_backlog

where 4<=N<=32.

Further, set the default backlog to 0, requiring listen() to be called
before we start actually queueing new calls.  Whilst this kind of is a
change in the UAPI, the caller can't actually *accept* new calls anyway
unless they've first called listen() to put the socket into the LISTENING
state - thus the aforementioned new calls would otherwise just sit there,
eating up kernel memory.  (Note that sockets that don't have a non-zero
service ID bound don't get incoming calls anyway.)

Given that the default backlog is now 0, make the AFS filesystem call
kernel_listen() to set the maximum backlog for itself.

Possible improvements include:

 (1) Trimming a too-large backlog to max_backlog when listen is called.

 (2) Trimming the backlog value whenever the value is used so that changes
     to max_backlog are applied to an open socket automatically.  Note that
     the AFS filesystem opens one socket and keeps it open for extended
     periods, so would miss out on changes to max_backlog.

 (3) Having a separate setting for the AFS filesystem.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 11 insertions, 8 deletions

diff --git a/net/rxrpc/af_rxrpc.c b/net/rxrpc/af_rxrpc.c
index 38512a200db6..a1bcb0e17250 100644
--- a/net/rxrpc/af_rxrpc.c
+++ b/net/rxrpc/af_rxrpc.c
@@ -33,8 +33,6 @@ unsigned int rxrpc_debug; // = RXRPC_DEBUG_KPROTO;
 module_param_named(debug, rxrpc_debug, uint, S_IWUSR | S_IRUGO);
 MODULE_PARM_DESC(debug, "RxRPC debugging mask");
 
-static int sysctl_rxrpc_max_qlen __read_mostly = 10;
-
 static struct proto rxrpc_proto;
 static const struct proto_ops rxrpc_rpc_ops;
 
@@ -191,6 +189,7 @@ static int rxrpc_listen(struct socket *sock, int backlog)
 {
 	struct sock *sk = sock->sk;
 	struct rxrpc_sock *rx = rxrpc_sk(sk);
+	unsigned int max;
 	int ret;
 
 	_enter("%p,%d", rx, backlog);
@@ -201,17 +200,21 @@ static int rxrpc_listen(struct socket *sock, int backlog)
 	case RXRPC_UNBOUND:
 		ret = -EADDRNOTAVAIL;
 		break;
-	case RXRPC_CLIENT_UNBOUND:
-	case RXRPC_CLIENT_BOUND:
-	default:
-		ret = -EBUSY;
-		break;
 	case RXRPC_SERVER_BOUND:
 		ASSERT(rx->local != NULL);
+		max = READ_ONCE(rxrpc_max_backlog);
+		ret = -EINVAL;
+		if (backlog == INT_MAX)
+			backlog = max;
+		else if (backlog < 0 || backlog > max)
+			break;
 		sk->sk_max_ack_backlog = backlog;
 		rx->sk.sk_state = RXRPC_SERVER_LISTENING;
 		ret = 0;
 		break;
+	default:
+		ret = -EBUSY;
+		break;
 	}
 
 	release_sock(&rx->sk);
@@ -591,7 +594,7 @@ static int rxrpc_create(struct net *net, struct socket *sock, int protocol,
 	sock_init_data(sock, sk);
 	sk->sk_state		= RXRPC_UNBOUND;
 	sk->sk_write_space	= rxrpc_write_space;
-	sk->sk_max_ack_backlog	= sysctl_rxrpc_max_qlen;
+	sk->sk_max_ack_backlog	= 0;
 	sk->sk_destruct		= rxrpc_sock_destructor;
 
 	rx = rxrpc_sk(sk);