can: gw: add a variable limit for CAN frame routings

To prevent a possible misconfiguration (e.g. circular CAN frame routings)
limit the number of routings of a single CAN frame to a small variable value.

The limit can be specified by the module parameter 'max_hops' (1..6).
The default value is 1 (one hop), according to the original can-gw behaviour.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

1 files changed, 40 insertions, 16 deletions

diff --git a/net/can/gw.c b/net/can/gw.c
index 37a3efb7cc9d..4216a80618cb 100644
--- a/net/can/gw.c
+++ b/net/can/gw.c
@@ -42,6 +42,7 @@
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/types.h>
+#include <linux/kernel.h>
 #include <linux/list.h>
 #include <linux/spinlock.h>
 #include <linux/rcupdate.h>
@@ -58,14 +59,25 @@
 #include <net/net_namespace.h>
 #include <net/sock.h>
 
-#define CAN_GW_VERSION "20101209"
-static __initconst const char banner[] =
-        KERN_INFO "can: netlink gateway (rev " CAN_GW_VERSION ")\n";
+#define CAN_GW_VERSION "20130117"
+#define CAN_GW_NAME "can-gw"
 
 MODULE_DESCRIPTION("PF_CAN netlink gateway");
 MODULE_LICENSE("Dual BSD/GPL");
 MODULE_AUTHOR("Oliver Hartkopp <oliver.hartkopp@volkswagen.de>");
-MODULE_ALIAS("can-gw");
+MODULE_ALIAS(CAN_GW_NAME);
+
+#define CGW_MIN_HOPS 1
+#define CGW_MAX_HOPS 6
+#define CGW_DEFAULT_HOPS 1
+
+static unsigned int max_hops __read_mostly = CGW_DEFAULT_HOPS;
+module_param(max_hops, uint, S_IRUGO);
+MODULE_PARM_DESC(max_hops,
+                 "maximum " CAN_GW_NAME " routing hops for CAN frames "
+                 "(valid values: " __stringify(CGW_MIN_HOPS) "-"
+                 __stringify(CGW_MAX_HOPS) " hops, "
+                 "default: " __stringify(CGW_DEFAULT_HOPS) ")");
 
 static HLIST_HEAD(cgw_list);
 static struct notifier_block notifier;
@@ -339,8 +351,23 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data)
         struct sk_buff *nskb;
         int modidx = 0;
 
-        /* do not handle already routed frames - see comment below */
-        if (skb_mac_header_was_set(skb))
+        /*
+         * Do not handle CAN frames routed more than 'max_hops' times.
+         * In general we should never catch this delimiter which is intended
+         * to cover a misconfiguration protection (e.g. circular CAN routes).
+         *
+         * The Controller Area Network controllers only accept CAN frames with
+         * correct CRCs - which are not visible in the controller registers.
+         * According to skbuff.h documentation the csum_start element for IP
+         * checksums is undefined/unsued when ip_summed == CHECKSUM_UNNECESSARY.
+         * Only CAN skbs can be processed here which already have this property.
+         */
+
+#define cgw_hops(skb) ((skb)->csum_start)
+
+        BUG_ON(skb->ip_summed != CHECKSUM_UNNECESSARY);
+
+        if (cgw_hops(skb) >= max_hops)
                 return;
 
         if (!(gwj->dst.dev->flags & IFF_UP)) {
@@ -371,15 +398,8 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data)
                 return;
         }
 
-        /*
-         * Mark routed frames by setting some mac header length which is
-         * not relevant for the CAN frames located in the skb->data section.
-         *
-         * As dev->header_ops is not set in CAN netdevices no one is ever
-         * accessing the various header offsets in the CAN skbuffs anyway.
-         * E.g. using the packet socket to read CAN frames is still working.
-         */
-        skb_set_mac_header(nskb, 8);
+        /* put the incremented hop counter in the cloned skb */
+        cgw_hops(nskb) = cgw_hops(skb) + 1;
         nskb->dev = gwj->dst.dev;
 
         /* pointer to modifiable CAN frame */
@@ -903,7 +923,11 @@ static int cgw_remove_job(struct sk_buff *skb,  struct nlmsghdr *nlh, void *arg)
 
 static __init int cgw_module_init(void)
 {
-        printk(banner);
+        /* sanitize given module parameter */
+        max_hops = clamp_t(unsigned int, max_hops, CGW_MIN_HOPS, CGW_MAX_HOPS);
+
+        pr_info("can: netlink gateway (rev " CAN_GW_VERSION ") max_hops=%d\n",
+                max_hops);
 
         cgw_cache = kmem_cache_create("can_gw", sizeof(struct cgw_job),
                                       0, 0, NULL);