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, 10 insertions, 0 deletions

diff --git a/net/rxrpc/sysctl.c b/net/rxrpc/sysctl.c
index d20ed575acf4..a99690a8a3da 100644
--- a/net/rxrpc/sysctl.c
+++ b/net/rxrpc/sysctl.c
@@ -18,6 +18,7 @@ static struct ctl_table_header *rxrpc_sysctl_reg_table;
 static const unsigned int zero = 0;
 static const unsigned int one = 1;
 static const unsigned int four = 4;
+static const unsigned int thirtytwo = 32;
 static const unsigned int n_65535 = 65535;
 static const unsigned int n_max_acks = RXRPC_MAXACKS;
 
@@ -100,6 +101,15 @@ static struct ctl_table rxrpc_sysctl_table[] = {
 
 	/* Non-time values */
 	{
+		.procname	= "max_backlog",
+		.data		= &rxrpc_max_backlog,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= (void *)&four,
+		.extra2		= (void *)&thirtytwo,
+	},
+	{
 		.procname	= "rx_window_size",
 		.data		= &rxrpc_rx_window_size,
 		.maxlen		= sizeof(unsigned int),