netprio_cgroup: limit the maximum css->id to USHRT_MAX

netprio builds per-netdev contiguous priomap array which is indexed by
css->id. The array is allocated using kzalloc() effectively limiting
the maximum ID supported to some thousand range. This patch caps the
maximum supported css->id to USHRT_MAX which should be way above what
is actually useable.

This allows reducing sock->sk_cgrp_prioidx to u16 from u32. The freed
up part will be used to overload the cgroup related fields.
sock->sk_cgrp_prioidx's position is swapped with sk_mark so that the
two cgroup related fields are adjacent.

Signed-off-by: Tejun Heo <[e-mail]>
Acked-by: Daniel Wagner <[e-mail]>
Cc: Daniel Borkmann <[e-mail]>
CC: Neil Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/sock.h

···
   *	@sk_ack_backlog: current listen backlog
   *	@sk_max_ack_backlog: listen backlog set in listen()
   *	@sk_priority: %SO_PRIORITY setting
-  *	@sk_cgrp_prioidx: socket group's priority map index
   *	@sk_type: socket type (%SOCK_STREAM, etc)
   *	@sk_protocol: which protocol this socket belongs in this network family
   *	@sk_peer_pid: &struct pid for this socket's peer
···
   *	@sk_send_head: front of stuff to transmit
   *	@sk_security: used by security modules
   *	@sk_mark: generic packet mark
+  *	@sk_cgrp_prioidx: socket group's priority map index
   *	@sk_classid: this socket's cgroup classid
   *	@sk_cgrp: this socket's cgroup-specific proto data
   *	@sk_write_pending: a write to stream socket waits to start
···
 	u32			sk_ack_backlog;
 	u32			sk_max_ack_backlog;
 	__u32			sk_priority;
-#if IS_ENABLED(CONFIG_CGROUP_NET_PRIO)
-	__u32			sk_cgrp_prioidx;
-#endif
+	__u32			sk_mark;
 	struct pid		*sk_peer_pid;
 	const struct cred	*sk_peer_cred;
 	long			sk_rcvtimeo;
···
 #ifdef CONFIG_SECURITY
 	void			*sk_security;
 #endif
-	__u32			sk_mark;
+#if IS_ENABLED(CONFIG_CGROUP_NET_PRIO)
+	u16			sk_cgrp_prioidx;
+#endif
 #ifdef CONFIG_CGROUP_NET_CLASSID
 	u32			sk_classid;
 #endif

net/core/netprio_cgroup.c

···
 
 #include <linux/fdtable.h>
 
+/*
+ * netprio allocates per-net_device priomap array which is indexed by
+ * css->id.  Limiting css ID to 16bits doesn't lose anything.
+ */
+#define NETPRIO_ID_MAX		USHRT_MAX
+
 #define PRIOMAP_MIN_SZ		128
 
 /*
···
 	struct cgroup_subsys_state *parent_css = css->parent;
 	struct net_device *dev;
 	int ret = 0;
+
+	if (css->id > NETPRIO_ID_MAX)
+		return -ENOSPC;
 
 	if (!parent_css)
 		return 0;