[PATCH net-next-2.6] net: add a recursion limit in xmit path

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mercredi 29 septembre 2010 à 20:43 +0200, Eric Dumazet a écrit :
> Le mercredi 29 septembre 2010 à 10:33 -0700, Jesse Gross a écrit :
> 
> > The tx lock has another use here: to break local loops.  With this
> > change, a misconfigured tunnel can bring down the machine with a stack
> > overflow.  There are clearly other ways to fix this that don't require
> > a lock that restricts parallelism, such as a loop counter, but that's
> > the way it is now.
> 
> Thats a very good point !
> 
> We could use a loop counter in the skb, but this use a bit of ram,
> or percpu counters in tunnel drivers, to avoid a given level of
> recursion.
> 
> /* this should be shared by all tunnels */
> DEFINE_PER_CPU(tunnel_xmit_count);
> 
> 
> 
> tunnel_xmit()
> {
> if (__this_cpu_read(tunnel_xmit_count) >= LIMIT)
> 	goto tx_error;
> __this_cpu_inc(tunnel_xmit_count);
> 
> ....
> 
> __IPTUNNEL_XMIT(tstats, &dev->stats);
> 
> __this_cpu_dec(tunnel_xmit_count),
> return NETDEV_TX_OK;
> 
> }
> 
> 

This can be handled generically in net/core/dev.c

David, if you accept the lockless patches, please apply this one
before ;)

Thanks !

[PATCH net-next-2.6] net: add a recursion limit in xmit path

As tunnel devices are going to be lockless, we need to make sure a
misconfigured machine wont enter an infinite loop.

Add a percpu variable, and limit to three the number of stacked xmits.

Reported-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
 net/core/dev.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 48ad47f..50dacca 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2177,6 +2177,9 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 	return rc;
 }
 
+static DEFINE_PER_CPU(int, xmit_recursion);
+#define RECURSION_LIMIT 3
+
 /**
  *	dev_queue_xmit - transmit a buffer
  *	@skb: buffer to transmit
@@ -2242,10 +2245,15 @@ int dev_queue_xmit(struct sk_buff *skb)
 
 		if (txq->xmit_lock_owner != cpu) {
 
+			if (__this_cpu_read(xmit_recursion) > RECURSION_LIMIT)
+				goto recursion_alert;
+
 			HARD_TX_LOCK(dev, txq, cpu);
 
 			if (!netif_tx_queue_stopped(txq)) {
+				__this_cpu_inc(xmit_recursion);
 				rc = dev_hard_start_xmit(skb, dev, txq);
+				__this_cpu_dec(xmit_recursion);
 				if (dev_xmit_complete(rc)) {
 					HARD_TX_UNLOCK(dev, txq);
 					goto out;
@@ -2257,7 +2265,9 @@ int dev_queue_xmit(struct sk_buff *skb)
 				       "queue packet!\n", dev->name);
 		} else {
 			/* Recursion is detected! It is possible,
-			 * unfortunately */
+			 * unfortunately
+			 */
+recursion_alert:
 			if (net_ratelimit())
 				printk(KERN_CRIT "Dead loop on virtual device "
 				       "%s, fix it urgently!\n", dev->name);